Hi,
I don't think it's related, do you publich the game as an exe or do you play within Unity?
if testers are using unity with a "pasted" project as opposed to a repository based sync, then it's likely that you are missing your project settings ( physics and time could have been tweaked).
also, are you sure it's not simply a question of frame based movement that on faster machine you get faster movement? it's always VERY important to have time based movement for a consistent experience regardless the framerate.
I would implement a simple framerate watched on screen and double check, if you find that on testers tat experience faster movement the fps is higher, then that's the issue.
Bye,
Jean