Yeah, it's easy to get odd results especially at close range where the arrow is flexing. You can get a situation where the point of the arrow breaks the first beam, but misses the second (due to flexing) the tail end of the shaft then breaks the second beam and you get 50fps. Or it happens round the other way and you get 500fps.
I had a lot of trouble making up a decent indoor lighting setup. Can't use fluorescent lights because of the flicker.
The draw length and loose make a huge difference too.
Del
(I got a new ball of string for Christmas
)