The reason one airsoft chrono may disagree with another is because they're mass produced in chinese factories and have no calibration at all. If the assembly person solders the second sensor 1mm further away on one then that chrono will read lower, likewise if the light 'gate' on another chrono is offset by a couple of mm in the other direction it'll read higher. That's just ONE factor which could affect the readings from a chrono, different batches of processors in different batches of chronos may perform differently.
One that always gets me is sites that chrono outdoors in direct sunlight... at black ops cribbs my 100% stock TM gun chrono'd 380fps five shots in a row on their chrono, obviously I called bullshit and realised that the sun was shining right down the tube, I asked to turn the chrono 90 degrees and try again because of the sun and surprise surprise 280fps.
Airsoft chronographs are not precision equipment and the people that operate them often don't understand entirely how they work. Unfortunately it's often a case of 'our house, our chrono, our rules', which I can fully understand. You get someone turn up to your site for the first time and their gun comes in hot on your chrono... do you A: take them at their word that it's fine, or B: trust your chrono?