DEXA and the Bod Pod are the most accurate methods for testing bf%. Problem is that these 2 dont exists every where, so its not the main one you hear about in text books and in study for testing people. Its usually calipers, and the Hydro if you can find that as well.
I'll tell you that those hand held ones are just GUESSES and are kind of junk really. We used 3 different ones once in school and they all read different. One read +3% lower than others, and one was higher.
And with those, go drink a bunch of water, come back and test and it will be different. Go gulp down another 8oz ow water, wait, test again, and different.
I dont like them at all, and only use them as a gauge as well as the bf% scales like the Tanita. Again, margin of error is about +/-3%