I think I mentally interpret "common" as "a member of a plurality category," which is to say in the same order of magnitude of commonality as the most common gr

I think I mentally interpret "common" as "a member of a plurality category," which is to say in the same order of magnitude of commonality as the most common group at a given level of detail. What you're describing, I think I would call "Not uncommon." Or, to put it another way, you shouldn't be surprised for any given case to exhibit it, but you shouldn't expect it either.