They could be using c02 Mags for their pistols and maybe the chrono is paint ball type, I heard that it give a higher fps rating, leading to your sig going over the limit.
Do the coke can test to see, not accurate but gives you a base to start.
Airsoft GI - Budget Friendly Soda Can Chronograph Test - YouTube
Skip to the important part.
Just buy a m90 spring and put it in if you are really eager to play CQB or just go to a field with limit of 400 fps.