How accurate is a scale that also measures body fat?

Hey there,

I just got weighed at the gym, and the scale also 'analysed' my body fat percentage. I am 18, 5'9", approximately 140lbs, and in pretty good shape. I have very muscular legs, slim arms and my waist is 65 cm, and I wear a small all around. I am a runner, and I do regular weight training as well as eating a clean diet (albeit nothing to extreme).

The body fat percentage measure said that my boyfat is 29.5 percent, which my trainer said fits into the overweight category. Now while I am well aware of the whole 'skinny-fat' phenomenon, it just struck me as odd in my particular case, since I work out regularly and don't have a lot of extra pudge on me (when I am standing straight, I mostly have to bend over to squeeze my stomach - the first two rows of muscles are visible, I am working on the next ones).

So my question is this: What is the margin of error on these scales? Ideally, I would like to get back down to 22% body fat (which I was two years ago, when I was doing off-trail alpine running mixed with multiple martial arts), but I doubt whether I am at 29% body fat - I also fit into the same clothes the same way I did when I was at 22. I have gained some, but certainly not that much?

Thanks in advance...