France does the same with regards to speed limits. There are also signs telling you the speed limit hasn't changed, or telling you to watch your speed, but without giving you the speed limit!

You are supposed to guess the speed limit from the size and shape of the road too. When I ask, almost nobody knows what is the speed of a road, they just wing it.

By law the speed limit has to be posted before an automatic speed trap (they are everywhere in France). Essentially training everybody that speeds limits are only for avoiding the speed trap "tax", but don't matter otherwise.