I'm not quite sure it works that way. MyBitcoin is a -shared- wallet. Yes, your address is unique to you but when you spend it can come from any address in the shared wallet. I imagine they tie the addresses for deposits to people and keep a database of how much each person owns. When you make a payment it can come off of anyone's address. This helps them keeps fees down as they can send "older coins". Just don't panic yet if you see some withdrawals off of your address that aren't yours...I'm kind of guessing though, I don't know how they have set it up.