Under these 2 circumstances will your available balance be less than the actual balance:
1. You have unexecuted orders
Please check if you have any unexecuted orders for the coins you intend to withdraw.
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 deposit (Probably the coin you intend to trade or any other coins) in your account. When your transaction has enough required confirmations, your deposit will be added to your account and the available balance will match its actual balance in wallet.