So, here is my LEGO Space Shuttle. Originally built in 1991, never disassembled (unlike the rest of my sets), so all parts are original and all were exposed to the same amount of sunlight.
However, the amount of yellowing is highly variable. The hinge for the manipulator arm (just back of the windshield) is like-new white, and the alignment pin for the arm (just in front of the green brick) isn't much different.
Two of the wing panels are much whiter than the others, and although it's hard to see, the hinged parts towards the back are two distinct shades of yellow.
What's the reason for this?