Cockpit yellowing/oxidization
 

Can someone explain to me what is causing this? I'm searching for a used boat and many I'm coming across, especially prevalent with nordics is the interior is getting yellow, with the exception of the cabin door and glove box area. I believe the doors are plastic, but when these boats were new, everything was basically the same shade of white. How is it possible that the interior gelcoat is yellowing, and the exterior is not?

https://cimg9.ibsrv.net/gimg/www.off...db7f27c983.jpg