So you might think that city property tax coffers would be getting some extra padding from real estate treasures such as a Kahala mansion that sold for $15.2 million in February, a hillside retreat in Makiki that sold for $12.3 million in May and a stylish residence fronting Waialae Country Club that sold for $5.7 million in May.

Well, think again.

Most Oahu property owners will see a modest increase in their 2016 property tax bill, as the value of all real estate on the island ticked up 6.1 percent over the last fiscal year based on assessments by city property tax appraisers. That’s in line with the rise for median home sale prices, and is partly based on sales up to June 30. But in the upper reaches of the island’s housing market where trophy properties shine, it’s not uncommon for city appraisers to value a home well below what a new owner paid.

Sometimes city appraisers can’t justify purchase prices as a “real” value. As a result, an owner’s property tax obligation can be based on land and building values far below what they sold for, resulting in less revenue for the city.

“Sales price and cost is not equal to value,” said Gary Kurokawa, the city’s deputy director of budget and fiscal services.

The difference for a multimillion-dollar home can amount to tens of thousands of dollars not flowing to the city.

For instance, an Illinois company bought an oceanfront Black Point mansion for $16.5 million from local real estate investor James Reynolds in 2013. At the time, the city assessed the property’s value at $6.7 million. Last year the city raised its assessed value to $8.7 million after the new owner spent roughly $1 million on improvements.

Property tax revenue from the estate at 100 Royal Circle, which includes an 8,626-square-foot house on a nearly 1-acre lot, rose to $51,934 last year from $23,637 in 2013. Most of the boost to the tax bill was due to a new residential tax classification for properties valued over $1 million and not occupied by the owner.

This year the city increased its tax-assessed value for the property to $9.5 million, which added about $5,000 to the tax bill, but dropped it for 2016 to $8.4 million, which is still almost half of what the owner paid two years ago.

In a more recent transaction, a company managed by Hawaii entrepreneur Kimberly Dey bought a home on an acre adjacent to the Honolulu Museum of Art Spalding House in Makiki Heights for $12.3 million in May. The city assessed the property’s value at $7.8 million for 2016, up from $6 million this year.

On Kahala Avenue a Las Vegas firm bought an oceanfront mansion in February for $15.2 million. Yet the city reduced its assessed value of this property to $11.3 million for 2016 from $14.6 million this year.

Another effective tax cut was made to a 4,244-square-foot home facing Waialae Country Club’s 11th fairway. In this case the city reduced its assessed value for 2016 to $4.3 million from $4.6 million this year. Technology venture capitalist Chong-Moon Lee bought the property in May for $5.7 million from a Chinese company that bought it in 2014 for $5.3 million from a Japanese company that bought it in 2008 for $4.7 million.

To be sure, there are instances in which the city’s assessed values are higher than what buyers recently paid, though property owners in these cases can appeal the city’s assessments. Owners of property that might be undervalued by the city, on the other hand, typically don’t complain.

Why the city might deem the value of a property to be lower than what the owner recently paid is sometimes influenced by not-so-apparent reasons.

Sometimes real estate is sold with personal property such as art and furnishings.

That was the case with the Kahala mansion that was sold in February. Shortly after the purchase, the new owner auctioned off items left by the seller in the nine-bedroom main house and a pair of four-bedroom guesthouses, including custom furniture, art and a samurai suit of armor. Auction proceeds totaled more than $70,000.

Another reason is that part of what someone is willing to pay for a residence may not be tied to anything tangible. Some might say this another way: that the buyer overpaid. This happens even with more ordinary homes, and can be a result of fierce competition among buyers, a buyer having a sentimental connection to a home, a buyer wanting to be close to family or a buyer making an uninformed purchase from outside the state without local knowledge or advice.

One big issue affecting tax-assessed values is the inability of city appraisers to visit every property.

There are about 295,000 private properties on Oahu which the city values at $228 billion in total.

Kurokawa, the city fiscal services official, said city appraisers will go look at a property only if it is sold, if a building permit indicates something was added or demolished, or if an owner appeals the city’s assessed value. In all other instances, assessments are made by looking at pools of sales by neighborhood to produce a kind of mass appraisal.

Even when visiting a property, city appraisers often don’t get to see much beyond external conditions from afar, so they might not have an idea of how good a view is, what the quality of interior finishes is and whether there are special features in a home.

“It makes it more challenging,” said Keith Yamashita, acting administrator of the city’s Real Property Assessment Division.

The mass appraisal method can yield good, efficient results in condominium towers, townhouse complexes and single-family tract home subdivisions where similar units can be grouped together and valued off a handful of sales. But in neighborhoods where quality, condition and custom features vary widely, mass appraisals can be far less accurate.

Brian Walther, president of local appraisal firm Uyetake, Uyetake & Associates Inc., said a good example is the Diamond Head area, where prices for homes sold over the past five years ranged between $1 million and $14 million.

Not looking at conditions such as the privacy and topography of a lot, access to the ocean, interior fixtures and quality of construction can significantly affect the assessed value.

“It’s kind of like the difference between meeting with a medical specialist for a health issue versus using a Web-based software to analyze your vitals in an attempt to diagnose your problem,” he said. “Although the results could be the same, or maybe even close, there is no substitute for a physical inspection by someone qualified and experienced. The same goes for real estate values.”

HARD TO ASSESS

Here are five condominium units in the Trump International Hotel & Tower showing purchase prices and tax-assessed values: