Plymouth Factory Service Manuals... something odd.

I am looking at the 1967 and 1968 Plymouth factory service manuals sitting side-by-side on my shelf, and I just noticed that the 1967 manual is over 1/2 thicker than the 1968 manual. This seemed particularly weird because the 1967 manual does not include Barracuda, which was issued as a separate supplement (maybe 40 pages). That would make me expect that the 1968 manual would have more pages than the 1967 version, not less. So my question is, what is missing from the 1968 manual compared to the 1967 manual, that it seems to have roughly 150 fewer pages?