Short Changed at Wal-Mart

The last four times (over about two months) that I've checked out at Wal-Mart's self-checkout stations, the auto-teller machine things have shorted me on change. It's never more than 5-15 cents, so I don't bother taking the time to get the difference. But now I'm wondering just how often this happens. The store I shop at has at least ten of these machines, which, when open, are always busy. Even at a couple cents an order, the store could still be making several hundred dollars a day in short-changing. It just makes me wonder, as this has happened consistently four times in a row.