To compute accuracy, you compare the moves which are made during the game with the best moves suggested by the engine. So, the engine will evaluate itself 100%, given its settings are the same during game and during evaluation.
You get 99.9something% when you evaluate one strong engine by using another strong engine (they're mostly aligned, but may disagree in small details), or when the engine configuration during the evaluation is different from the configuration used in a game (e.g. engine is given more time to think).
To compute accuracy, you compare the moves which are made during the game with the best moves suggested by the engine. So, the engine will evaluate itself 100%, given its settings are the same during game and during evaluation.
You get 99.9something% when you evaluate one strong engine by using another strong engine (they're mostly aligned, but may disagree in small details), or when the engine configuration during the evaluation is different from the configuration used in a game (e.g. engine is given more time to think).