Why don't anime seasons have a standard amount of episodes?
This seems to happen in most regions. The typical reasons I've seen given are:


shows don't all start on the same day or even the same week
sometimes airtime slots are occupied by sports, seasonal, or other non-regular programs
networks may choose not to broadcast new episodes on holidays with historically low viewership
shows are sometimes only given an awkward number of slots like SuperGirl here.


I would also hope that some studios choose to show more or less episodes to ensure they have the right pacing and tell the whole story without an obnoxious amount of filler.

I imagine networks know pretty far in advance what their lineup will look like. Pilot episodes or other proposal material is submitted to the network far in advance for them to review before agreeing to broadcast. It is also important to remember that a large portion of every day's slots belong to re-runs so cancellations just call for more of those.