Paul,
I'm curious if these whiter stickers are still browning over time, just much more slowly than the rest. I would think they're all subject to the same chemical reactions since they're all made of the same materials.
I think the key issues are what the sticker is made out of and the environmental conditions. Not all tan backs are exactly the same. There are glossies, and a couple of different qualities of stock amongst the non-glossies.
For a given type of sticker, long term storage in hot/humid conditions will accelerate the deterioration.
But yes, give it a thousand years and they will likely all be toned, much like all newsprint will eventually yellow.