I have found only a small difference in AFMA between wide and zoom for most of my lenses.
It's also worth noting that the difference you found is small enough that it's well within the margin of error. That is, on any given shot, the AF system itself is going to vary by at least +/- 1, which is the difference you list for your lenses.
Personally, with your findings, if I could only have one adjustment (rather than the two you get with modern bodies and zooms), I'd split the figures and forget about it.
That's a great thing about FoCal -- it plots the results. And you can plainly see that pretty much any number in that plateau area is going to produce the same real-world results -- and that sweating over neighboring values in the middle of the sweet spot is an exercise in futility.