I can tell you that I also don't exactly understand it, but it is what it is : )
I think this might be the possible reason
https://bitcointalk.org/index.php?topic=1057998.0I think it is possible, if we count from the 1st March of 2014 to now (1st May of 2015)... we obtain = 436 days. 1 activity point * day, so the account has 434 as activity (there is not wrong in that situation).
31 weeks * 14 points = 434 activity points.
Secondly if some one Post 1 post in every 14 days then his other 13 points are bank that he can get them later by posting.