I’m not sure it was necessarily a matter of Richter not preparing enough or not meeting a certain standard of perfection… I think every performance, for him, was a toss up between whether to behave, or let loose. I think ‘let loose’ usually won out.
That said, he was a completely different type of artist than Michelangeli. I doubt he ever much cared if his playing was note perfect. For him, the highest standard meant getting the music right more than the notes.
Or whatever. Who the hell knows. Dude was a mystery.