Just look here https://bitcointalksearch.org/topic/forum-rankspositionsbadges-what-do-those-shiny-coins-under-my-name-mean-178608. You have everything you need to know there. Need help with something else maybe?
I linked the OP to that page on this page haha, that's where I got the formula. I looked it over, and it seems that you have to post within that 2-week period for it to count toward your activity. That's the only plausible explanation.
Yeah that's it. I wasn't clear with ranking system here when I was starting here too and now I'm Sr. Member, almosta year now.