How Accuracy Works...

Your accuracy stat helps determine if you hit. You make an attack adding in your accuracy bonus, versus their defense.

But in the case of a move like hypnosis, where you have a natural harder chance to hit, you follow this rule of thumb: For every 5% miss chance in the game, -1 to your attack roll. So, an attack that hits 60% of the time, recieves a -4 to hit here.