Switch methods on the same data and watch one add to the balance while the other adjusts to it.
Credit sales $400,000 × 1% = expense $4,000.
Existing allowance $650 + expense $4,000 = new allowance $4,650.
$4,000
$4,650
The starts from the income statement: a fixed rate times credit sales, added to whatever the allowance already holds. It never looks at the old balance.
The starts from the balance sheet: it sets the ending allowance directly, and works backward to whatever expense gets there. The same facts can produce two different numbers, and both are correct under their own method.