Here is a note to people who use my Buy Sell Ads WordPress plugin. I recommend that you set cache expiration to 0 to make your statistics register correctly with the Buy Sell Ads network.
This is because Buy Sell Ads network is now also tracking the number of times the ads themselves get requested (not just the tracking pixel). If the ads are cached on your server, then they are not going to count in your Buy Sell Ads statistics whenever it is being displayed (lowering your impression count).
I am working on ripping out the caching mechanism right now, but for the time being, you should just set cache expiration to 0 in the Buy Sell Ads WordPress plugin settings.