Well funny story, some twenty something years ago I actually worked on an election cycle volunteer infra thing in France, and living in Paris which is department 75 and therefore 750xx the prefecture being 75000 I assumed it was neatly hierarchical 75004 won't be far away from 75003 (true)... The French thing being orderly and rational.

I didn't need much precision so truncating seemed an easy way to group stuff.

Oh the surprise. I never again made such assumptions, let's just say I should have gotten a clue from Corsica being 2A and 2B.