Because it's the low hanging fruit in the modification detection game. The ATF wants something on paper they can stick you with that's easy to check like a dowel rod to check barrel length. JMO, it may not be the reason they haul you off but a "22lr" suppressor with a .475" bore will end up being just one more charge on the pile you have to clear, a little more time on the sentence or maybe the chit that ups you into the next felony bracket.
This is the line of thought that I'm following.
In the event that you've garnered the ATF's attention, and they do take a look at your toys to make sure they are properly registered... take the scenario I posed earlier: burglar dispatched with a .45 pistol and a ".22" can.
It seems to me that they would look at the Form 1 saying it's a .22 can. They would look at the engravings on the can saying it's a .22 can. Then they would see the .5" bore that has .260" of clearance... that's four times the often recommended .060".
There is a strong case, in my mind, to be made that either
a) The bore was increased, which means it's a new can in the ATF's opinion, requiring a new Form 1 and tax stamp.
b) The information on the Form 1 was intentionally incorrect.
Both run afoul of the same tax code with the same $10,000/10 years penalty.
You might win in court, but by trying to be cute and registering the can as a .22, you've set yourself up for a fight.
The original thread over on The High Road centered around building a .30 cal can, and potentially being able to use it down the line on a .45-70. The caliber / bore disparity in that case is significantly smaller, but I still don't see what advantages there are in registering the can as a .30 cal when you think you might want to use it on the .45-70 down the line. All I see is a potential risk that could easily be mitigated by registering it as a .45 can.