Some people who withdrew to their wallets still have enough to cover their withdrawal in their account. For example, if I had 1000 bitcoins, made another 2000 bitcoins during the crash, then withdrew 500, after the rollback, I'll have 500 left. (The original 1000 - the 500 I've withdrawn).

Some peoples' accounts might be 'negative' because of withdrawals. Mt gox will have to cover these losses if the users don't return the funds. I'm sure it isn't that much.