You may have purchased investments associated with that account. If that's the case, AceMoney added value of these investments to your account balance on the main page. Account page only shows a cash portion of the balance, so it is different from cash + value of investments. You can make AceMoney ignore value of investments and display only the cash balance of the account on the main page by unchecking "Add value of investments to account balance" in Tools->Options.

Another possible reason is different currencies. If your account currency (configured in the Account Properties dialog) is different from the default currency (configured in Tools->International Settings...), then balance on the account page is shown in that account currency, but balance of the account group on the main page is in default currency. Note, that even on the main page balance of the account is always displayed in the account currency. AceMoney uses default currency only for the group balance and the total balance.