At least that's what Gox reported. When they made the statement it was maybe true (for one day or two, next to the bubble)
Is it still true (if it was true at all)?
Next: Withdrawal restrictions. Deposit 1 Million $ at Gox? One transaction. To withdraw your money completely, you have to make many small transactions over a longer period of time.
The amount of money going into Gox being higher than the amount of money withdrawn in any given period of time is not very surprising.
If the quoted statement is true then there are dozens of Millions at Gox just hanging around in stand-by. Which means: No interest rates and not quickly withdrawable. I can't really imagine why big investors would do that, but maybe that's just me.