In this game, my opponent blundered his queen. It happens. However, on the last move, before resigning, he also blundered his knight. Curiously, the game did not consider this a blunder as after losing his queen, his chances were very low already. So, in this case, the difference between his position and his position after dooming his knight were so close, that despite it being a bad move, the engine did not bother to mark it as one.
This then gives some insight, an inaccuracy, mistake, or blunder are drops of the ratio by some significant mutually exclusive margin- a low drop, medium drop, or large drop respectively.