The SMP client uses a bonus points system to determine PPD, and it is based on how quickly you return the WU from the time you downloaded it. Thus, if you shut the client off at night, your PPD will suffer because a lot of time has gone by without any progress on the unit. PPD also goes down when you're using the computer vs when it is idle, and gaming/cpu intensive things will kill ppd. Make sense?
Also, always let it run 3-4 frames before you look at ppd so you get a more accurate reading.
