The Available Balance is the account balance you actually have use of. It does not include deposits of checks which have not been finally collected. It does include the days deposits and withdrawls that have been provisionally posted to your account but are subject to final posting at the end of the day processing.
The Current Balance or book balance is the account balance that includes all deposits and withdrawls that the Bank has processed against your account on the prior banking day.
Because the Current Balance is the balance in your account at the close of the prior business day, it does not reflect items that are currently in process.
The same way Available Balance takes in consideration deposits not finally posted to the account, it also takes in consideration withdraws made against your account. When the current days withdrawls exceed the current days collected deposits your Current Balance will exceed your Available Balance. It is important to remember that the Available Balance is the account balance you have use of.