The historic opening/closing balances in Float are calculated from the current 'Balance from' figure.

Float works by taking the balance as of today and working backwards, adding/subtracting the reconciled transactions. As only the reconciled/matched transactions are imported, any unreconciled items won't show in Float and won't be included in the historic calculation.

This can lead to the historic balances displayed in Float differing from those shown in the accounting software, however once things are fully reconciled, the calculation ties everything together.