hathorcat couldn't have said it better! I think there are many things that go into determining how white a pony is regardless of age, as well- particularly how they were stored and how they have been handled over the ages. If a white g3 is handled constantly by a child with dirty hands they'll tend to look just as dingy as every other pony would, and if they are not stored properly they could possibly get any dingier, much the same way a g1 could.
I have a g1 Baby Dangles (who was just given an oxyclean bath not too long ago) who is currently whiter than any of my white g3s, dingy or not! So I guess if you wanted to name fingers and point names at what causes variations in white, I'd say it boils down to: age, type of plastic used to create the pony, how it has been stored, how it has been handled, how the pony was made in the factory, and probably how it has been cleaned, as well!