One mistake there in the deposit address and how it actually works.  Someone can correct me if I am wrong, but the deposit addresses are hot wallets for the exchange.  Even though a deposit address is unique to an individual, the funds are transferred off into other wallets from that address.  So if you deposit 5 btc to address x, and then proceed to send 2 btc to another address, the source of the transfer most likely won't be address x, but another hot wallet.  I believe most exchanges operate this way.  When someone else from the exchange sends 5 btc, it may actually come from your deposit address. 

Someone please double check I'm posting the correct information.  Please expand also if needed.