So I did the math on my Magcargo and according to the in game Pokedex, it has base HP of 50. However when my Magcargo hit level 100 it had 20 extra hitpoints. It should have been 295 Hitpoints if it really was base 50 but instead it had 315.
Math: Base HP - 50
50x2+10 = 110 + 100 for level + 22(IV) + 63(EVs) = 295
I decided to check bulbapedia which shows its base HP as 60 instead of 50. This gives exactly 20 extra hitpoints.
So while the Hitpoints are correct in game, there seems to be a visual bug showing the incorrect amount in the in-game pokedex.
Link to Bulbapedia showing Base 60 HP from Generation 7 onward: https://bulbapedia.bulbagarden.net/wiki/Magcargo_(Pok%C3%A9mon)#Base_stats