I play an ideal move in a solved position, and you play a less-than-ideal move in response, I have already gained an advantage.
That assumes the less-than-ideal move stays within your preparation. If, on the other hand, you dismissed that move during preparation because of its sub optimal evaluation then I have succeeded in getting you out of prep and into a middle game with a small disadvantage but potentially a lot of play. If I can then steer the game down a very sharp line then your small advantage means a lot less due to the dynamics of the position.
That assumes the less-than-ideal move stays within your preparation. If, on the other hand, you dismissed that move during preparation because of its sub optimal evaluation then I have succeeded in getting you out of prep and into a middle game with a small disadvantage but potentially a lot of play. If I can then steer the game down a very sharp line then your small advantage means a lot less due to the dynamics of the position.