Battery backup clocks

what good is having a clock with batteries in it to save the time when power goes out. If it just decided to come back on with the wrong time. I just recovered from another power outage. Its currently 06:04. But my clocks are telling me its 06:30, 06:16, 06:28. What gives? Is this a manufacture defect? Or do the battery weild some great power that forces the clocks to speed up?