The JPMorgan Chase & Co. unit responsible for at least $2 billion in losses on credit derivatives was valuing some of its trades at prices that differed from those of its investment bank, according to people familiar with the matter.

The discrepancy between prices used by the chief investment office and JPMorgan's credit-swaps dealer, the biggest in the U.S., may have obscured by hundreds of millions of dollars the magnitude of the loss before it was disclosed May 10, said one of the people, who asked not to be identified because they aren't authorized to discuss the matter.

"I've never run into anything like that," said Sanford C. Bernstein & Co.'s Brad Hintz in New York, ranked by Institutional Investor magazine as the top analyst covering brokerage firms. "That's why you have a centralized accounting group that's comparing marks" between different parts of the bank "to make sure you don't have any outliers," said the former chief financial officer of Lehman Brothers Holdings Inc.