write-off: revenue vs cash collected decrease, accounts, collected, understand, revenue

chunty

vol2 pg383 says "write-offs will reduce the gross accounts receivable balance" -- this I understand.
"We can infer a $0.025 billion write-off from the decrease in the allowance for doubtful accounts" -- I thought allowance for doubtful accounts was used to reduce accounts receivable. Shouldn't a write-off be the case where such allowance increased, unless the write-off refers to the allowance itself? Because of this I am not getting the formula on the next age.