Why is the soldering surface of this pogo pin slightly rounded?
The curved bottom will allow for betting wetting when using solder paste, as it gives more surface area on the bottom where the solder paste actually hits. 

This is less important when hand soldering as you generally use far more solder and it will wet up the sides of the pin, but in large scale manufacturing they use much less solder paste and you want to maximize the surface area  exposed to the solder paste. If it was flat the bottom would lay flat against the pad when it is reflowed and give far less surface area. 

In addition the solder paste is very viscous and and will hold the pin in place, so the pin doesn't actually need to balance on a flat bottom.