It's fair that a cheap test might not be super accurate, but then why are they stating figures like "13%" if the margin of error is at least 10 percentage point

It's fair that a cheap test might not be super accurate, but then why are they stating figures like "13%" if the margin of error is at least 10 percentage points? That's just plain misleading. When my doctor asks how tall I am, if I'm 6'1", then 6'1" is a good answer. 6' (or even better: about 6') can also be an acceptable answer, as it's an approximation. I wouldn't say I'm 6'8", and then try to rationalize it by saying my tape measure is cheap, and anyway it's still right to about 10%. Reporting what you don't know can be just as important as reporting what you do know. It doesn't matter how good your laboratory method is if you can't report results in a way that doesn't mislead customers.