The counter is screwy.
I sometimes log in for days and it doesn't show up.
Yes I noticed it too, I have 11 days but I spent literaly months logged in, maybe it counts only when you post something, then stops after a determined time.
I don't think you actually have to post something, but the general idea is right; it counts as long as you are actively using the forum (like browsing through the threads), and stops after a set time of inactivity.