If a machine measures your weight to be 100 lbs, and the machine is not accelerating (at any rate, not accelerating quickly), and there are no extenuating circumstances, and the machine is well-oiled and frictional forces are neglible, and if the weighing machine measures correctly (that is, it doesn't measure the same weight differently depending on how much weight is already on it)... then I would say it's probably exactly right.
If the machine is more realistic, I guess it depends on how the machine's made.
I imagine most real machines get your weight a little low, due to frictional forces. Plus, they probably sell more scales that way.