BPs are really good at storing up waste materials for a while.
It makes it really difficult to get an accurate weight on them. One day you're good and the next there's a giant poop in the corner...there goes a lot of that weight.
Honestly I barely weigh mine as well. The only reasons I weigh is if I'm tracking the growth of a potential breeder that's almost there, if I'm going to be selling the snake, or if the snake actually gives me reason to be concerned.