Why is my opening balance different from my bank?

Most common cause: unreconciled transactions between the import cutoff date and your first bank feed sync. Check Reports → Bank reconciliation as at the cutoff date — anything outstanding will be a gap. Other usual suspects: timing of in-flight cheques, BACS payments not yet cleared at the bank, or fees the bank applied that you haven't booked. Reconcile period-by-period to isolate which month the gap appeared in, then drill into that period's outstanding list. Once each period agrees, the running balance will too.