wow, that can be 6% off, thats quite terrible.
however, as soon one has abs showing and lower than that, its not so hard to say anyway, and anything where theres no abs whatsoever in sight is too fat.
from say 13-18% men look pretty much the same, the calipers would be very useful in that range
It's not quite that bad. The +/- whatever % numbers are usually the extremes of some sort of statistical distribution, meaning it's possible but pretty unlikely to be 3% off in either direction. Most of the time it's probably off by 1 or maybe 2% with good equipment/conditions and a good tester. I don't think it's the "average" error.
Also the +/- doesn't mean it could be off by 6%, it means that the error is bi-directional (i.e. it could be too high or too low) but the measurement itself will be off by an absolute error of within 3%.
I could be off on the +/- 3% thing though, I haven't looked this up for a while.