Basically a heavier BB fired from a gun leaves the barrel with
more energy than a lighter BB fired from the same gun. As such, you should cronograph with the heaviest BB you intend to use that day and not the .20s they throw in your gun from their maraca mag at the crono table. Almost all sites will use a lighter BB than what you actually go and shoot, and this is where the creep gets in.
The problem itself stems from sites not knowing how - as
@Iceni says - fluid dynamics works with relation to a force being exterted on an object when its surface area (i.e. the outside of the BB) is the same, but its mass/volume ratio is different (i.e. heavier BB but still the same size). The problem is exacerbated by HPA and GBBRs, where there's usually a lot of air being shoved down a barrel that can frequently be 'volumised' beyond what an AEG is. This can lead to people thinking HPA players are intentionally shooting hot when in reality they're using their .30s and .32s and are hitting you with more energy that usual. Again, a lot of the time HPA users are using heavier BBs than AEG users, and this too exacerbates the problem.
In addition, heavier BBs lose their energy less readily than lighter ones during travel for the same reason. That means they hit harder longer away, and is why you should always use the heaviest BBs that a) your hop can lift consistently and b) you're happy to throw money down on. They will go further.
Another way to solve this would be to ask for the weight you're using, then measure the fps (with that weight) and obtain the muzzle energy in Joules. Sites should really be cronographing in Joules instead of ft/s, but that confuses a lot of people who just want to throw plastic at each other, and that's fine. What I don't find excusable is bad operators letting non-malicious players take the flak for their lack of knowledge and ability to control the game. I've even been to sites where they ask 'what weight are you using' and have the applicable ft/s in their head to aim for - this also works, but is less elegant solution than just taking the weight and speed and spitting out your muzzle energy. After all, the new restrictions are based on the muzzle energy of your gun in Joules, and not ft/s/fps.
Below is a chart that allows you to calculate Joules. Remember the limit in the UK for automatic stuff is actually 1.3J now. Anything over is considered a fully-fledged firearm. Here the chart says 1J is the limit.
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