I'm pretty sure that the manufacturers grade the boots. The initial system was thought up by Brian Hall, a guide working for Scarpa, to try and clear up some of the confusion that existed before there were any gradings at all.
The idea's since been supported by manufacturers generally, but the BMC's article on crampons - see www.thebmc.co.uk - doesn't even mention crampon / boot grades.
For an outline of the system see:
http://www.scarpa.co.uk/crampon/index.html
As they say there, it's just a set of guidelines rather than an absolute rule. There are so many variables - boots soften with age, larger sizes can be more flexible than smaller ones, because the sole units are under greater load with more leverage, some crampons simply don't work with certain boots - see the Scarpa Freney for an extreme example.
The bottom line if you're buying crampons and unsure, is always to take the boots along and get the crampon fitted to them by an experienced shop where the staff know what they're doing.. If you're in doubt, be cautious - having a crampon fall off on steep ground is unnerving and can be dangerous, so only use clip-ons if you're certain they boot is designed for them.
