The covers are made by the same people in the same place. So i doubt there is any difference
My wife bakes lots of different types and flavors of cakes. She uses the same pans, the same oven, the same kitchen, and most of the same ingredients, yet they all come out very different just by slightly altering a few SPECIFIC ingredients.
Polymers and resins work much the same way. You can create multitudes of different outcomes using the same basic ingredients. You don't need different chemicals to produce them, just altering the ratio in formulation can make all the difference.