<< Why is it that the posts per day average seems to fall faster(when not posting) vs how slow it increases(when posting)? >>
To answer your question qualitatively, when you're not posting you're averaging 0 posts per day, which brings your average down faster than when you are posting and you're just maintaining your current average or slightly above it, unless you go gung-ho and start neffing up the wazzu.
To answer more quantitatively, by my calculation, you've been registered 496 days. In that time, you've posted 1267 messages. This works out to 1267 / 496 = 2.55 Average Posts Per Day. Now, let's say you wanted to bring your average up by one post per day during the next month. Let's call how many posts you need X. So then the calculation would be X / 503 = 3.55. Solve for X, and you get 1786 posts!. So, if you want to bring your average posts per day up by one in the next month you need 1786 - 1267 = 519 new posts in the next month. During that time you'd be averaging 519 / 30 = 17.3 posts per day.
So, you'd have to average 17.3 posts per day over the next month just to bring your average posts per day up by one!