As with everything we do, there is a subjective element to setting limits, but it's definitely not as arbitrary as you are suggesting. Maybe they reduced one limit because there were too many accidents, and maybe they increased the other because they finally got the signal pattern working as intended.

Risk assessment is incredibly complex. It might be perfectly reasonable to drive 110km/h on a given road most of the time, but frequent use by large farm equipment could necessitate a lower speed. Or, maybe adjusting traffic on road x decreases accidents on road y.

We are still learning how to produce vehicles that reliably compensate for variables like friction, or human reaction time. The implications of even these two simple things seem to be completely lost on most drivers: with a tiny bit of rubber touching the asphalt, we happily drive around in inconceivably heavy vehicles at rates where it's very easy for an event to begin and end before we even suspect something is imminent.

While I'm here: turn your lights on when you start your car, turn into your own fucking lane, always move over if someone is behind you in the fast lane even if you think you're going "fast enough" (someone could be bleeding out, seriously), don't pass people on the wrong side, and finally: stop trusting the meat in your head so much, our brains fuck up all the time, so in addition to driving defensively wrt external factors, consider how you can set yourself up to succeed if something unexpected happens internally.