It is just a guide so you have a point of reference to track your progress.Nobody actually knows what 3%, 6%, 10%, or 20% body fat actually looks like.
Absolutely right. The only way to know with 100% certainty how much body fat you actually have is to take it out of your body and weigh it (not possible while alive). If you feel healthy and you (and others) think you look healthy, I don't think it's anything to worry about.