Under these 2 conditions will your available amount be less than the actual amount:
1. You have unexecuted orders
Please check if you have any unexecuted orders for the coins you intend to withdraw. If so, you can cancel the orders to recover your available withdrawal balance.
E.g. If your wallet balance is 5 BCH, meanwhile you have an unexecuted sell order of 1 BCH for BCH/BTC trading pair. In this case, there will be 1 BCH frozen in your account, unavailable for trading, and your available amount will be 4 BCH, less than actual balance.
2. Deposit confirmations are not enough
Please check if you have any pending deposits that haven’t reached the required confirmations for withdrawal (Probably the coin you intend to trade or any other coins) in your account. When your deposits have enough required confirmations, they will be added to your account and the available withdrawal balance will match its actual balance.