I agree. Lately, I've done a few shorts with my margin, and the amounts always seemed messed up unless you just go at least one overnight without ANY open positions, long or short. Usually it updates, but I think, looking at my "cleaned up" balances this morning, that it may still be a little outdated.
And unfortunately, I've run into a couple of margin calls as a result. Nothing huge and quickly fixed by closing a position, but still, I don't like getting those margin calls.