Does anyone know why the bubbles on certain characters particularly wave 1 are often much more yellow than wave 2 and 3 figures.

Where the bubbles on Wave 1 MOC'S a different quality?

I've noticed Catra seems to be the worst MOC for yellowing, with DT, Castapella and Frosta not too good either.

Wave 3 MOC's - I don't think I've seen any yellowing