I agree that max-vg as a term used for labeling is idiotic, but for personal usage and used as a term to strive for/goal(i.e. what's your prefered ratio?) I think it's perfectly fine and accurately described, i.e. no matter if your nic is PG or VG based and how much flavoring is used, then no extra PG added(actual PG, not counting flavors and/or nic), hence max-vg obtainable with your current ingredients.
The differences in unthinned VG is because of it's highly hygroscopic nature and from the start also about 0.5% is allowed in the USP I believe(normally ranging from 0.04 to 0.3'ish based on COAs i've seen), though from there, more can easily be leached from packaging/bottles/caps/headspace... The purity of USP VGs generally only deviates about 0.2%(glycerol content) from COAs i've seen...