Is there a reason that dev has a preceeding dot, while a and rc don't?

I think that's just how https://peps.python.org/pep-0440/ does it:

[N!]N(.N)*[{a|b|rc}N][.postN][.devN]

Ah, I knew I’d seen it somewhere…

There was a specific reason, IIRC it had to do with string sorting of versions matching their semantic ordering