The balance of an account is what you have in the account.

The current balance of an account is calculated by adding to the initial account balance, all paid transactions to current date. The balance report is only available for periods already passed, does not make sense to view balance for the future. 
The forecast of an account instead is what I will have in the future. The forecast at the end of the year of an account is calculated by adding to the initial account balance, all transactions paid and unpaid till the end of the year. The forecast report is only available for future periods.