As of 8/23/2010, we have changed how installs are tracked for use in the retention analysis. We now only consider new installs, whereas previously we considered all installs received in a given day, week, or month, regardless of whether the user had installed the application previously. If you only send us application install ('apa') messages for new users, you should not see any change in your retention numbers. If you have additional questions about this, please contact support.