I wouldn't doubt that the markings are not exaclty correct. One of the reasons you hear of semis getting stuck under bridges is that truckers know all bridge heights are marked as being shorter than they really are, so they add like 6" to a 1' to what the sign says, next thing you know, they'r letting air out of the tires and smearing vaseline on the roof of the trailer.
I have tried two different garages marked 6'7" and my truck would have barley scraped the bottom of both signs. I parked ins a garage marked 6' 8" with no problem a couple weeks ago.