I thought they changed that a while ago, so that if you go short against a deposit of BTC or long against a deposit of USD you "borrow" (without any fees) from your own funds before you take any swap. I remember an announcement saying that they'd realised that the way it was set up you effectively got more leverage by holding your deposit in the same currency as you were borrowing, and that the change had evened it out so that it didn't matter which one you started out holding.