other people, your not the only person using the exchange.  you put in x amount currency they can supply x amount in BTC because they have it from the people that put in x amount BTC to trade for x amount of currency.

This can be the delay they all talk about in their TERMS that it can take 1-3 days or more to complete transaction so there is a 1-3 days or more delay to aquir said funds of any currency.