As Craig mentioned, this probably has something to do with a bonus money.
Casumo mixes both real money and bonus money together and allows you to withdraw in the same ratio that you were given the bonus. For example 100% bonus would allow you to withdraw half of all the money on your account, until the wagering requirement for the whole sum is reached.
Obviously, if you manage to wager the whole wagering requirement, then you can withdraw all the money from your account.