I agree--it looks like there are a lot of people using different methods to measure.
On another note, I am watching the posts on the topic of POA's and how much they can carry. I've read a lot recently, and there seems to be two camps. The first states that POA's, like other horses, should not carry more than 20% of its own body weight. The other camp says that ponies have shorter backs, thus enabling them to carry weight more easily; and if the pony has denser bones (such as Arabians), that furthers their ability to carry weight as well. Since POA's are bred from Shetlands, Appaloosas, and Arabians, their bone density ought to be (in my opinion) better than average.
This being said, I'm still uncertain if my POA is a tough little girl with power in those muscles, or she's just a little horse and should carry a smaller load.