PPD or Points Per Day is a floating calculation and based on the current last "frame". Essentially it determines how long it took, full throttle, to do a frame which is from Point A to Point B. Based on that time frame it calculates that you would get X PPD if you let it go running as it was when it was calculated. As soon as you shutdown the process the timeline alters because now you've not gone start to finish full throttle. It then calculates the remaining time in the day and adjusts the points lower because you cannot get as many since the process as stopped, if that makes sense
PPD is a nice thing and gauge and really can only be trusted if you get a work unit, that can be completed within 24 hours and you let it do the entire thing at once without interruption of causing the process/gpu to fluctuate in % used. Seeing how Windows constantly adjusts CPU/GPU usage as you do other things such as opening a web browser or retrieving email it's can be a constant changing value. Only the reported work units completed in a day's time is calculated and other sites can give you those point values.
Also, different work unit "cores" or the application that crunches the numbers give different point values. Typically GPU work units offer higher point values at shorter runs because of the raw processing power the GPU can harness however while doing so do think you can play a game or even watch YouTube as when mine is cranking full bore YouTube skips video and even locks up at times. It's all how much you can dedicate at that time and just like vaping any work you can do is a great achievement!
~Icky