Everyone's "normal" is different, depending on their genetics and "normal" diet/exercise regimen. Maybe they should list bf% along with arm measurement, which would make the arm measurement more meaningful.
I know what you mean, but then most people's body fat distribution varies. When I put on weight it always goes to my hips first and so I don't tend to have fat arms, though my waist could be a little chubby. Other people could be the opposite. I'd say the easiest way to distinguish would be something similar to what benz just said, cold flexed, below 10%, 10-20% or above 20%? Most people won't know their exact BF % anyways and also, even if they did, there's a good chance it's wrong