What was seen at the 93O was the improvements Dorian made from 91 to 92. He could have come in at 257 in 92.
You're discounting a whole year more of training he achieved from 1992 to 1993 though. Even if he had not overdieted in 1992, he would NOT have stepped onstage in 1992 and looked like he did in 1993. He was 275 pounds 3 weeks out from the 1993 Mr. Olympia. If anything, he may have overdieted for the 1993 Mr. Olympia, which shows just how much he improved from 1992 to 1993.