That's the limit for continuous rated power. The motor's frequently have 600W-750W of peak power output, and can legally use this much for short amounts of time (usually seconds, like accelerating from a stop; but often also for going up a steep hill for several minutes).
The point in distinguishing the different classes is about where the bike should fit into the ecosystem. Should it ride on the shoulder, interacting with pedestrians and slower bikes, or should it ride on the road, interacting with cars and motorcycles.
It doesn’t matter how much riding it takes, it matters how fast and controlled it is moving compared to the other traffic in that class.