It is kind of odd wording though to speak of "a physics subreddit" since it seems like it would make sense for there to be only one ("the physics subreddit" instead of "a physics subreddit"). That's not how things are though with /r/math, /r/maths and /r/mathematics existing all at the same time despite focusing on the same topic.
Do you really think so? I imagine if you aren't familiar with Reddit from context a subreddit just means a topical discussion board, and if you are familiar with Reddit your default assumption would be there isn't only one subreddit for topics unless they are really niche like a mid sized city or sports team.
I suppose if you are only vaguely familiar with Reddit you might it assume it's like a vBulletin setup where only a handful of subforums are set up by admins rather than users.
