People, this is mathematics. Like it says, posts per day means an average of your total posts divided by the number of days you've been on AHS since you registered. It's obviously directly proportional to how much you've posted and inversely proportional to the ammount of days you've been on AHS. If you've been posting a lot less recently, the rate will obviously drop, because the time is still running (duh). There isn't, as far as i know, any obvious relation to editting posts or quick replies and the such, because they won't alter the overall post count, unless you've chosen to delete a post.
Honestly, does this parameter even matter at all? I just rarely check it to see how much MIA/busy/etc have I been recently, compared to past years (I think back in 2003 I had an average of 10 posts per day XD) LOL You can use it to check which initiates have been massively posting everywhere recently too LOL
I can also use it to calculate how many days have I been registered to AHS: 10,688/5.82 = 1836 days
It's been over 5 years already LOL