Differences in notation between different fields abound. It used to be (probably still is) the math department didn’t talk to the engineering department, so people would reinvent basically the same solution in different contexts. Now people maintain the notation for internal consistency, instead of switching to “universally standard notation”.

Basically, historical consistency trumps cross-disciplinary consistency.

I mean in RL we use n consistently. It's all over the Sutton and Barto book. I'm looking at it now.