Between yesterday and today, there has been a reduction in bitcoins by 0.00214616 (not much, but it's more the principle).
I have contacted them, and they gave the reply that I originally posted ("the balance varies according to the prices that are floating").
What you're experiencing is exchange specific and does not really have much to do with Bitcoin itself. The exchange can change your balance at any time, although that would make them a scam. Keep in mind that Bitcoins owned on a exchange are not yours, since you don't own the private keys to the addresses.