>>2691312750% Z did not have IVs or EVs with 183 HP at level 50. If anything, when the Zygarde goes perfect it only adds the base HP of the forme it's going from.
Consider that 10% went from 125 HP to 287 HP, a difference of 162.
50% went from 183 HP to 291 HP, a difference of 108.
It can be likely that 10% gets the boost from 50%'s base (108) + half (54) to get the boost of 162.
