The beginning of the fall auction season always produces dramatic headlines about the art works that achieved record-breaking prices and those that bombed, yet beyond the hype, all auction results can be misleading. During last week’s contemporary art auctions in London, for example, the top lot sold was Eric Clapton‘s painting by Gerhard Richter, Abstracktes Bild (809-4). The painting went for £21.3 million ($34.2 million) at Sotheby’s, way above the estimate of £9 million to £12 million, and set a new auction record for a living artist.

In total, Sotheby’s made £69.9 million ($112.1 million) of sales from its contemporary and 20th century Italian art auctions in London last week, close to its top estimate of £74 million. That looked pretty fantastic on the face of it, yet without the sale of that one Clapton painting, the auction house would not have even reached its low estimate of £54 million.

Auction house sales figures are misleading for another crucial reason. After any auction, the final sale result published by the auction house for each lot is not the hammer price for that work, or in other words, the agreed sale price reached when the auctioneer’s gavel finally comes down. Instead, the auction house also adds on the buyer’s premium that was paid to that hammer price amount.

That buyer’s premium, the amount of commission that all successful bidders must pay the auction house, is pretty hefty. In New York, the big global auction houses currently charge 25% of the hammer price up to $50,000, 20% of the amount above $50,000 to $1 million, and 12% of the rest, although those percentages and thresholds vary from country to country, depending where the sale is held and the currency it is held in.

Why does this matter? Well, it obviously inflates the overall price achieved for each lot that it successfully sold. But because auction houses only include the buyer’s commission in their final sales results, and not their pre-sale estimates, it also makes the difference between, say, the estimated price for a Picasso painting and the actual price it sells for, look a lot better than it actually is.

Auction houses say this practice is a well-known convention, but it’s actually a big deal for anyone trying to extract price comparisons from auction market information, which as I mentioned recently, is the only real source of art price data out there.

Without the buyer’s premium, last week’s results start to look a little different. Take out the buyer’s premium and Eric Clapton’s Richter was sold for £19 million, not over $21 million. Christie’s combined sales of post-war, contemporary and Italian art in London last week, which raised £41.2 million ($62 million), compared to its estimate of between £36.6 million and £51.9 million, was pretty weak already. Take out the buyer’s premium and the auction house barely reached its low estimate.

The buyer’s premium issue makes the record-breaking prices trumpeted by auction houses and reported in the press pretty misleading. With November’s impressionist and modern art and post-war and contemporary sales in New York fast approaching, that’s definitely something to keep in mind. Here are a few of the star lots up for auction in New York next month:

Sotheby’s is going to try to follow its success selling Eric Clapton’s Richter in London with the sale of Abstraktes Bild (712), Richter’s first painting from 1990, in its evening sale of contemporary art on November 13. The estimate? Over $16 million.

Also in its November 13 contemporary evening sale, Sotheby’s will offer No.1 (Royal, Red and Blue), painted by Mark Rothko in 1954, one of the paintings the artist himself selected to appear in his solo show at the Art Institute of Chicago the same year. The estimate: $35 million to $50 million.

In its November 14 sale, Christie’s will offer Andy Warhol’s 3D silkscreen painting Statue of Liberty, produced in 1962. The estimate is also in excess of $35 million.

Of nine Picasso paintings offered in Sotheby’s impressionist and modern art evening sale on November 5, Nature morte aux tulipes, painted in March 1932, is expected to fetch $35 million to $50 million.

The star lot of the New York impressionist and modern art evening sale at Christie’s on November 7 will be one of Claude Monet‘s famous water lilies series Nymphéas, painted in 1905. The estimate is between $30 million and $50 million.

Other than collectively indicating that when an auction house isn’t really sure what a super famous painting will sell for, it sticks a price tag of $30 million to $50 million on it, the thing that all these estimates have in common in that they do not include buyer’s premiums. Whatever Christies’s or Sotheby’s say these paintings fetch come November, the hammer price will be lower.