Comment by awesome_dude

The rolling balance is a "projection"

Your bank statement has the event (A deposit or withdrawal) with details, and to one side the bank will say, your balance after this event can be calculated to be $FOO

The balance isn't a part of the event, it's a calculation based on the (cached) balance known from the previous event.

Further, your bank statements are (typically) for the calendar month, or whatever. They start with the balance bought forward from the previous statement (a snapshot)