Some thoughts....
- Some plastic holders that yellow do not seem to degrade cards stored within them.
- Yellowed borders could be from a home where people smoked.
- Topps often used two grades of card stock with their sets. One seems to stay white no matter what and the other gets dingier with time.
Not sure that's what's happening with your examples i.r.o items 2 and 3 but they could be contributing causes.