Interesting; I thought I'd read that even the very best players only average ~90% accuracy, whereas the best engines average 99.something%?
Edit: Even lower-level cheated games are rarely 100% accurate for the whole game, cheaters usually mix in some bad or natural moves knowing that the engine will let them win anyways. That's why analysis is usually on critical sections, if someone normally plays with a 900 rating but spikes to 100% accuracy every time there's a critical move where other options lose, that's a strong suggestion they're cheating. One of the skills of a strong GM is sniffing out situations like that and being able to calculate a line of 'only moves' under pressure, so it's not nearly as surprising when they pull it off.
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).