There is a lot more to being healthy than bodyfat percentage and there are very few accurate measures of this all to commonly used benchmark.
electricity takes the path of least resistance so the hand grip test measures the resistance offered by your body as it travels up one arm, across your shoulders and down the other. It then uses a bunch of preset templates based on what gender height etc. you have told it you are and estimates the entire rest of your body. My measure on one of those is generally 8 or 9%, and I haven't been 17 for a while. The stand on scales do the same but measuring resistance in your legs. I usually score around 15% on these.
I am genetically designed to be a beanpole, this combined with a ridiculously high metabolism and what most consider excessive training means that even my 4,500 to 5,000 calorie a day intake doesnt' tend to elevate my bodyfat.
As jrahien says you are the best judge of how healthy you are regardign bodyfat. If you are constantly lethargic, depressed or depleted, eat more, if you have plenty of mental and physical energy and are generally upbeat your body is fine.
Guidlines are for the average person. I know many of them and many far from it. The talent is looking at the bigger picture.