1) The New Posts count shows you how many posts have been made since you last clicked "Mark All Forums Read". So if you never click that link, the number will forever increase as long as there are people posting here. One reason I changed it from red to dark blue was to make it less distracting for those of us who never mark forums read.
2) The New Posts count shows how many posts have been made, not how many threads have been posted to. So the number of threads listed will not match the number of new posts.