Originally when the new profile pages came out, the "They are active in these communities" list of subreddits on your profile was based on total karma. So the ranking would be entirely based on how many points you have per sub.
This produced some mild and harmless inaccuracies. Like, for example myself. The first six months after I created this account, I participated almost entirely in /r/TodayILearned. I have almost 160k comment karma in TIL, even though I haven't really participated there in a year and a half or so.
So TIL showing up as #3 on my "active" list isnt exactly accurate, but it sort of is.
By far, I prefer that sort and wish I had an option to choose it. Instead, my "you are active in" list constantly fluctuates based on any subreddit which I've made three recent comments in.
It's rather annoying, as it portrays an even less accurate overview of my use of this website than the previous "you are active in" sort.
ninja: TLDR, made four or so comments on Gallowboob's profile page and it became one of my "you are active in" subreddits.
That's actually hilarious and clearly broken!