>>58037383Mega stones are stones imbued with Infinity Energy, you could, in theory, create more of them by deliberately blasting stones with Infinity Energy, because that's how they were incidentally created by the Ultimate Weapon.
I'm assuming that's what happened between XY and ZA, because that makes the most sense to me, it also has the potential for humor with characters valuing older mega stones far more than 'fake' mega stones, not realizing that all mega stones are inherently artificial.
The much dumber alternative is that Lysandre's botched mass culling somehow created more mega stones than AZ's successful mass culling.
The dumbest alternative is that this isn't going to be addressed at all, meaning we just have to assume no more were created since the Ultimate Weapon was first fired, more were just found over time and mega stones are retroactively not as special as we were told.