Let’s Go works a little bit differently from other main-series games. Let’s Go doesn’t have EVs, but instead has what’s called AVs, which are similar, but distinctly separate. EVs can only ever raise a stat by 63 points, while AVs can raise a stat by a whopping 200 points. It’s actually impossible for a level 78 Mewtwo to have 464 HP with EVs, but AVs are able to push it that far. However, AVs also only exist in Let’s Go, and will disappear if the Pokémon is sent to SwSh.