Q:  Why does the number of "temperate Earths" rise steadily
         over the life of the mission, while the number of "hot Super-Earths"
         stops rising after a short time?


       The "hot Super-Earths" have very short periods, so they will transit
          every few days.  The telescope will observe 5 or 10 or 20 or more
          transits within one year, and so (nearly) all such systems will be 
          noticed quickly.

       The "temperate Earths" will have orbital periods similar to the
          real Earth, around one year long.  That means that PLATO will
          not observe 3 transits by any of these systems until at least 
          two years have passed, and systems with slightly longer periods
          will take more time to accumulate at least 3 transits.

          In addition, some of these temperate Earths will have transits
          which are BARELY visible, so it might require more than the 
          3 observed transits for the team to conclude that the signals
          are real.  These marginal systems will be marked as "official
          detections" after 5, or 6, or 7 transits.