Quoted where?
The current CRT published dimensions issued in August 2022 are:
8' 9" from Brentford to Long Buckby (by which I suspect it means the bottom of Buckby locks i.e. Whilton)
6' 3" from Long Buckby to the top of Camp Hill locks
The historic (and therefore presumably legally binding) headroom from Long Buckby to Camp Hill is 8' 0" so whether the 6' 3" relates to some temporary works, some issue that has arisen since 1967 or the construction of something below 8' 0" I don't know. It could relate to the width vs height issue but CRT dimensions don't appear to make that consideration in general.