Step-by-step explanation:
It came from nowhere. It makes no sense to add up the balance numbers. To illustrate, let's use a different example:
Adding up the money you spent, and you get $500. Add up the balances, and you get $1000. But why would you add the balances? The 300 in the second line is included in the 400 in the first line. You can't add them together. You'd be counting the 300 twice.