There are a couple of different reasons these balances might be different if you check the below areas:
Check that any old bank nominals (accounts) haven't get a balance in them and by mistake have had monies posted to.
Check on the bank statement through your online banking that the bank has the correct balance and has calculated the monies coming in and out correctly.
Check that all the items reconciled are showing on the bank as going through to make sure that nothing has been missed.
Check the nominal doesn't have anything posted as the new period hasn't been reconciled in the Bank Reconciliation.
If there is still a difference after checking these areas, please raise a new case online and reference the title of this article.