Good info.

If beer, mead and wines was only alcy and water, then maybe the gap bewteen the factors might be affected by the types of alcys. I bet one could look up the common alcy's that are produced during fermentation and I wouldn't doubt that the SG of each (in a purer form) would be different. If that is so, the blend of those alcys would affect the overall SG of the mix. And I wouldn't be surprised if that blend can also change at different stages during the feremnetation

And it might have something to do with the extra stuff (minerals and the like) in there as well. With that in mind, perhaps a mead would have a different factor than an equal origanl/finish gravity wine. Similarly for beer.

I'm just speculating at this point but wouldn't be surprised if something like the above was the reason.