Two things jump to mind:
- Great players can correctly tune a deck to a given meta. I’d recommend listening to Andy Markiton’s post-Champs interview with SMIP as an example of this; he gets into some interesting discussion of how he arrived at his 75.
- In-game, they don’t really make mistakes. It’s not about brilliant, unseen lines of play so much as always correctly (and quickly!) evaluating their options and the consequences of each game action.