Is there a reason? Do the dots in the release numbers confuse things? Or is it purely historical?

I think it goes back to Debian using “Toy Story” characters for releases - they’re in the same bed here as Ubuntu (I’m running “bullseye”). I’m not sure how it started but it’s too cute for no gain. At least the docker images are tagged with both so you don’t need to remember whether “jammy” is an LTS or not.