Billionaire John Magnier got a windfall from the sale of an Amedeo Modigliani nude at Sotheby’s in May. The auction house was not as fortunate.

The 1917 “Nu couche (sur le cote gauche)” fetched $157.2 million, an almost sixfold increase in value since 2003 when the Irish bloodstock tycoon bought it. Although Sotheby’s heralded the price as the highest auction result in its 274-year history, the painting was sold on a sole, prearranged bid. In the end, Sotheby’s didn’t keep much of its $18 million commission.

Things got worse a month later in London, when Pablo Picasso’s 1932 “Buste de Femme de Profil (Femme Ecrivant)” sold for about $10 million less than Sotheby’s pre-sale target.

The artworks were thrust back into focus on Monday, when Sotheby’s said its second-quarter auction commission margin fell to 14.1 percent from 16.3 percent a year ago. Investors watch the figure closely because “gross profit is really auction commission dollars,” said David Schick, managing partner at Consumer Edge Research.

The shares slipped 5.6 percent to close at $49.93, the most since February.

Michael Goss, Sotheby’s chief financial officer, attributed the decline in commission margin partly to the sale of two large guaranteed paintings. They weren’t identified during a call with investors and a Sotheby’s spokeswoman declined to comment.

The paintings show how unforgiving -- and fickle -- the auction business can be.

On the face of it, Sotheby’s and Christie’s sell staggering amounts. Sotheby’s tallied $3.5 billion of sales during the first six months of 2018, up 22 percent from last year and just shy of its best result at the top of the market in 2014.

As the market heats up though, auction houses are forced to give up more of their commission to win trophy artworks.