If Zygarde can have a form change mid-battle into something with a different base HP, why can't Megas have different base HP?
Muh patterns
They can
>>33159234
Because they didn't think of it before Sun and Moon. Did you actually expect Ganefreak to be smart?
Because Zygarde cells = HP.
They can but why would they? Keeping the same base hp makes most sense, and I would usually prefer of the stats were distributed elsewhere