I'm no authority on this, but it seems reasonable "They" would likely shoot 5-10 shots on a prototype and look for varience, top speed, and average speed for a given pellet weight. They'd definitely err on the side of caution, and de-tune the gun as necessary. May explain why the box says 495fps, but the results are usually lower.
Manufacturers would have guidelines they follow to meet the standards of wherever they're shipping to. It'd be interesting to actually see the testing standards, especially what pellet weights are being used...
Maybe Canada will finally just revise the laws governing airguns, and use fpe like our brethern in England.