http://www.wantchinatimes.com/news-subclass-cnt.aspx?id=2013...
In general the way it works for a non ponzi scheme setup is that another user on MtGox is selling, and you are buying, so essentially if you buy, your account balance of coins goes up and at the same time the seller's balance goes down. Keep in mind though, no actual bitcoins moves because MtGox is holding them all in their account. It's exactly like what happens when you transfer money to someone who banks at the same bank as you. The money moves from one account to the other on the ledger, but really no money moved at all.
The key here was that somebody basically compromised gox's security rather than some random user's security, and managed to put coins in a gox account without actually depositing any bitcoins. This was why rolling back was a justified action (albeit the hack shouldn't have been possible to being with).
From a programming perspective, it doesn't really make any sense to make real Bitcoin network transactions for every buy and sell order; an internal database can keep track of all that way better. Arguably, they could steal all your Bitcoins if they wanted to anyway, so it doesn't really matter how they handle the internals.
Also there are people on the thread claiming they traced the wallet that had those coins.
"On June 20th at approximately 3:00am JST (Japan Time), an unknown person logged in to the compromised admin account, and with the permissions of that account was able to arbitrarily assign himself a large number of Bitcoins"