The site PawPaw_n_Gram pasted that info from was from 2007. Woodhall has additional sites stating the same bogus information, the most recent being 2011.
I know for a fact that the Illinois height information is wrong. Since none of the other information matches current nationwide publications for designated highways I would assume that all of the height information is wrong.
As stated before, if you don't want to hit an overpass, or get in a situation where you have to try to back up because you didn't see the sign in time, then there are two things you can do:
(1) get a Rand RV GPS which will warn you in advance of a height restriction.
(2) pre-plan your next days route by consulting a Trucker's Atlas.
What you don't want to do is count on seeing the sign in time for all the overpasses. As already has been pointed out, due to re-paving and other issues sometimes the exact height is incorrectly reported on signs, or maybe some juvenile misfit stole the sign, whatever the case it is pretty reckless driving down a secondary roadway with no idea what height restrictions lay ahead.
But, just like all the other posts on the subject, this is just my opinion.