Designing a good scale that will be cheap enough that people will buy it in large numbers is not easy.

Would you be happy with a scale that was 99% accurate?

Sounds pretty good.

Now figure that if your true weight were 250# that scale could read anything within a five pound range and be within spec.

As a previous post says, you have to hunt carefully to find a scale that has good precision (gives you the same reading every time). The best thing is to only weigh yourself periodically (say once a month) so that the amount of weight you lose is (usually) more than the variability of the scale.