The enforcement threshold is much lower in other countries, but the posted speed limit is more reasonable and matches a safe speed for driving.

The Japanese law doesn't allow criminal enforcement until 30km/h over the posted speed limit, so the posted speed limit is very slow but enforcement for 10km over is non-existent and limited for 20km over (you'll only get a ticket if you are unlucky).

There are many reasons for the gap between the posted limit and enforcement threshold in Japan, but the TL;DR is that the real-world speed that everyone drives at is generally 20km over the posted limit.