Saddle rail breaks often During the last few years, I've had a broken saddle rail every year or so. It always breaks just in front, or just behind the clamp. I wonder what the reason could be, and how I can avoid this.

Here are two pictures:



Some information:


I'm ~75 kg (165 lbs), so not exactly heavy
I'm only cycling on paved roads, no bumps whatsoever, roughly 12 km (7.5 miles) / day
My bike is an ordinary city bike, Shimano Nexus 7 speed, roller brakes, size 37-622 tires, no suspension
Most (affordable) saddles over here in The Netherlands are by a single brand 'Selle Royal'. Therefore I expect the size of the rails would be the same for all of them, and shouldn't be an issue 
Two years ago I replaced the clamp, because the old one looked worn out. This seemed to solve my problem, but today, I found it broke again


I found this question (Saddle Rail Failure?) but was wondering if there could be other reasons. 

UPDATE 20 aug 2018

I spoke to the guy at the bicycle shop. According to him it is probably caused by a combination of the rails not being centered properly, and maybe also the way I ride over (speed)bumps. So Andrew, you were right after all, I guess (although I would have to wait another year to be sure ;-) ).
The clamp in your seat post is unusually narrow, and this generates higher than normal high bending forces on the seat rails. A seat post with larger clamp should solve this problem.