Are you frustrated about inconsistency across the same unit at the difference times, across the same model but at different venues, across the same brand, across different brands, at testing with AEG/GAS/HPA?
I've been the same boat as you! I highly doubt I'll buy a non quick change spring weapon again, especially since I just bought a bunch of springs.
Again this is probably a lack of an industry wide body; If for example, the govt may be putting through legislation about the power (joules) of airsoft guns in the near future, but its rather pointless if they don't legislate on the reliability of the measuring tool. I'm sure they have a list of approved 'tools' but that does not mean they will mandate it.
I suspect that one of the reasons, in that legilsation, that the joule limit they specific results in around 20fps more than 350 fps with 0.2g is exactly to cope with inconsistency at the chronographs, the same with their upper limit for single shot, falls above 500 fps with 0.2g.
I get the impression that chronographs, for airsoft purposes, are not made to the same tolerances as those which measure much higher velocities.
As there is no governing body, there wont be a "measurement must be with this tool" - and thats good tbh, gives each brand and equal chance and makes them affordable for individuals.