Different material compositions. I used that example since they were in the same car and in similar spots in the car, the storage conditions could be eliminated as a variable.
I’ve seen examples of low mileage vehicles that were stored exclusively in climate controlled environments. Some years and models of GM vehicles will experience headliner sag due to the backing breaking down no matter what. Yes, the climate controlled storage may have slowed it down a bit for some….but probably not as much as we’d like to think.
Another observation I’ve made are the front shoulder belt guides. 88-90 are of a different style and material than the 91+. The 91+ style always turned a sort of chalky looking white with age, regardless of storage conditions. The 88-90 style may have faded a bit with tons of UV exposure, but almost always remained nice and black.