That's the way it should be. All posts, right from the first one you posted when you first joined, should show the current cumulative total. Your first post doesn't remain on 1, and so on. They all show the same number, so that you or anyone else looking at any post you made, will know instantly how many you have posted in your time here, in total.
Some forums do have a slight update problem in that whilst the post you just made will show the correct total, some older posts lag a few behind, and it takes them time to update. This forum seems OK in that regard, though.
ETA: I see Matt has already explained this principle. Sorry.