If you check a bag, you need to have it weighed. Working in a grocery store in high school, I remember how the scales weren't always calibrated - people pulling on them, kids swinging on them. The baggage scales at airports get an even bigger workout, staff stepping on them, kids weighing themselves, all shapes and sizes of bags being thrown on them.
Jerry Butler, with the Standards Division of the North Carolina Department of Agriculture, confirms at that airport scales can't always be trusted. "They will get off because people are dropping 50-pound bags on that thing hundreds and hundreds of times a day," Butler said. http://www.wral.com/pack-light-and-use-bathroom-scale-to-avoid-airline-baggage-fees/11798351/
If hes going to check a bag for an upcoming trip, DH always weighs himself on our bathroom scale, then gets on again holding his suitcase. The difference is, of course, the weight of the bag. We've never had over weight bags so haven't run into the problem of paying for excess baggage, but I've seen other people asking the airline rep to weigh their bags on another scale. Like the produce scales in your grocery store, the baggage scale should start at zero. Paying for checked bags is expensive, at least make sure that the weight of your bags is correct before you hand over that credit card.