The information gives Don a creepy feeling up his shell. He's thought about being vegetarian, sure - but he's never done it. His life doesn't allow for that kind of luxury. The idea that in some other universe, a different version of himself actually made that choice, makes reality seem to shift a little.

He suddenly finds that he doesn't really want the other sandwich.

"When did you move in with April?" he asks, partly to change the subject, and partly to find out in what ways his life could have been better, if something had been different.