@irontoenail - Another thing that might throw scales off is using them on carpet rather than a hard, flat surface. And scales aren't just used for people who are obsessing over losing weight.
There are people who need to gain weight and use scales to track their process. There are people who are relatively small and even half a kilo either way is important, so they need a really well calibrated set of scales.
I personally prefer to use the scales at the doctor's though, since they must be relatively accurate. They use one of those old fashioned scales with weights which I don't think need to be calibrated all that often.