Someone just laundered their money through your compromised account. Congrats! You're in trouble!
You should be ashamed of yourself, fudding this poor confused guy.
The likely explanation is this:
Your address at coinbase pays into a shared wallet controlled by coinbase. While all funds sent to you are assigned to your account, the actual coins can be used to pay for other people's transactions. You'd see the same sort of thing with a deposit into mtgox.
When you withdraw funds they'll pay you back with some other coins entirely.
