to IIS Daniel!
Well you are correct in thinking that the focal plane of the telescope is fixed relative to the location of the primary mirror, but the other variable is the location of the focal plane of the eyepiece (EP) relative to the EP barrel. This can and will in general vary from EP to EP, which is why different EPs need different focuser travel.
That's pretty good service to get a barrel extension free of charge just because you happen to have EPs that don't play friendly with the scope.