So there's a Zygarde Core with a red mark and a Zygarde Core with a blue mark, well I guess that explains why Zygarde 100% Form has a blue and red half. If I'm allowed to theorize, it'll be revealed that individually the Zygarde Cores are only able to maintain a 50% Form. If we go to using computer terms, Zygarde 50% is a normal computer while Zygarde 100% is one with duel core and multi-processing (thus allowing Zygarde 100% to do more things than Zygarde 50% since it now has two brains running the system instead of one). Also with their being two Zygarde Cores that means there can be two Zygarde 50%, meaning ideally no Zygarde Cell is being wasted from non-use (since Zygarde 50% is Zygarde's preferred form, 10% is an emergency form).