Quite often collectors will pay more for the first model, even if later versions are better to shoot.
It’s not always downhill. I think the heavier barrel of the Vulcan Series 2 makes for a nicer gun than the original, even more so in Series 3 guise with the better trigger. FWB 124/127 went from plastic trigger to metal, and a better front sight.
Anyway, the Blue Book isn’t gospel, more of a guide. It has some errors and omissions in it.