The game have a real time battle so some modifications on the officials stats are neccesary.
Before the 2.16.1, the HP base stat was multiplied by 50, now is multiplied by 15.
The HP EVs was fixed because they weren't taken into account.
Catch Mewtwo is a challenge for much players. But with the new version, battles are faster and pokemons without EVs are weak, consequently Mewtwo too.
A boss system isn't here for add a diffculty to catch Mewtwo so Mewtwo HP stats move from 106 to 600.
Much abuses in PVP for xp gains so it's removed.