For SNES, Nintendo probably had different manufacturers - but this experience of mien relate to the EuroJap model, the US one might have only one provider and be more consistent.
Just browse eBay for used SuFami and on them yellowing is all over the place; some only get sunfaded on some parts, other turn all yellow.
Reverse pattern :
Yellowed all over (even the pads!)
Yellowing on the bottom of the case clearly inducate that yellowing is caused by different things. Certainly, UV and heat will break down the fire retardants faster, but there have been reported cases of NOS, never opened SNES being opened and found yellowed.
If I remember well, Nintendo wanted to focus on the US market so they set up their own making line for it, so they mgiht have relied on only one manufacturer; they picked up the Japanese shells for Europe mostly because the "release peak sales" was past them in Japan. Still they had difficulties to provide both, the SNES sales in Europe were stronger than expected, and getting parts from different manufacturers and mixing them when assembling consoles (there is really only two different parts between a Jap and Euro console : the cart flap on early models (SuFami One says "Insert cassette here" the Euro one says "PAL version") and the front part with the logo)
There is one exception, the French model doesn't have RF out, so Nintendo molded a "hole less" part :
Rear of a French model that suffers from "brittle plastic" even without clear yellowing :
Edited by CatPix, Sat May 18, 2019 8:24 AM.