I was under the impression that the bitcoin people deposit are actually there. But I just found out about the "open.btc" on the bitshares network. They apparently peg the value of a btc to "open.btc", which is what you own... not actual btc. Then I read that all exchanges do this. So when you deposit btc to poloniex, you actually own poloniex.btc or whatever they call it.
I didn't get far enough to understand why they do this, but it is an issue I was not aware of until just recently. Not sure how comfortable I am with that. But I only keep hot currency there anyway. 80% of my crypto is in cold or mobile storage.