Players do learn offenses and defenses besides the ones you run and practice, and there's some speculation as to what causes players to learn certain non-practiced formations better than others. I'd say luck probably plays some role, and that's probably what happened here. If you could assign a numerical value to O/D ratings, let's say an F is anything below 20. Now let's say your walkon started with knowledge of 19 in the press but 15 in M2M. Well maybe his M2M rating is now 18 (and thus still an F) but his Press randomly increased one point to a 20 which is the baseline for a D-.
I'd say wait until you've had 5, 10, 15 practices and see what the results look like. Without any data to back me up I'd still say you'll see that your walkon will be much stronger in M2M than Press.