Oversized is a state thing. The feds just set where oversize starts. If the states choose to set a higher standard they are allowed to.
While being 3 or 6 inches over is no big deal they have to draw the line in the sand somewhere. If 6 inches is allowed then why not 9.
In fact the feds are already giving you 6 inches on some roads. Legal width is 96 inches but if you stay on a piticular set of roads they allow 102 without a permit.
Permits are also a good thing especially when dealing with height. The state will set the route based on known clearances.