I have never conducted this experiment, but my commone since and other skills would have to ask:
Have you ever used a laser level?
Have you ever tried to focus a 400mm telescopic lense?
Was the wind perfectly calm?
How do your arrows fly with a broadhead?
The point I'm trying to make is, you could be seeing wobble that is not noticable to the naked eye, and could only be noticable under a microscope, being the light against the dark, which is probably not nessessary.
How are the arrows impacting, left, right?