Yes, some of these speedlimits seem arbitrary, making very little sense. There is a curve outside my village where you can do 60; in the summer people miss the curve multiple times per week (luckily that results in them just driving of the road on a flat bit of land; not into a crevice). Far far less sharp curves where you could drive 60 easily, you are allowed 20-40 suddenly... Same for some high quality, wide roads where you can do 50-60 while outside the town limit; 80-100 would be fine there. The gossip (and I have no clue if there is any truth to it!) about those limits is that all places where a serious accident happened at some time in the past, no matter what caused the accident, had the limit slashed and it's not going back up.