Depends. I don't think they should measure height from the top of the head. I think they should measure QB height from the eyes. Not actually kidding. Look at the forehead of Peyton Manning. You could play a movie on that thing. He's half an alien with that dome.
Seriously...not joking... there's got to be very close to 6 inches from his eye line to the top of his dome. That's a lot.
Now look at Jared Goff... his "eye level offset"...what else would you call it... looks closer to 4.5 inches.
I'm using an estimation based on sizes of facial features among other clues from other pictures.
I guy could be 6'8", but if he's basically got a stove top hat for a cranium, it really doesn't help him see over the OL.
Not that Case Keenum or Drew Brees have eyes on the top of their heads and it's all the same or anything, but all 6'6" guys aren't equal...