What else is new? A lot of firearms related law is very vaguely worded, such that the entire field is very much a gray area subject only to ATF interpretation of laws.
For example, a limit on barrel length. How, exactly, do you define that? ATF uses a method where they basically prescribe how to measure it, and the result of that measurement constitutes the length, but it is still somewhat variable. The way they do it is by closing the bolt, and then lowering a measuring rod from the muzzle until it hits the bolt face; the amount that is concealed is the length of the barrel.
Fine, but depending on whether the gun is cocked or not, the firing pin might or might not be protruding from the bolt face, and if it is protruding, then your measuring rod will hit it first. If your barrel is already near the legal length, this can actually push it over into the illegal territory. There is a persistent rumor (for which I haven't seen any evidence, so take it with a grain of salt), that it is precisely how ATF agents "busted" Randy Weaver when he modded a shotgun for them in a sting operation.
Basically, the laws as they are, turn ATF into an agency that is exceedingly powerful, because they can reinterpret a lot of laws by executive fiat in such a way that what was perfectly legal yesterday suddenly becomes illegal today - and the infringement of those laws is a very serious felony punished by many years in prison, and a reasonable suspicion of it is considered a legal justification to have a SWAT team raid your house.