It may be because the bank has wrongfully manipulated the order in which it processes your debit card transactions. Some banks have been hit with class action lawsuit because they processed payment of debit card transactions from largest to smallest amounts in order to maximize any overdraft fees they could assess. See the Bucks blog on the New York Times: Chase Agrees to Settle Lawsuit on Overdraft Fees and Debit Overdraft Fees, While Voluntary, Are Still Steep.
