Because there are speedpoints and distance points. If you are not finished you only have distance points. So at first the valid start, without penalty points, is always the one with the highest score. When you finish it could change to the one with a high start penalty that gives more speed points. And then it could change again as the day factor (ratio between speed and distance points) depends on the number of finishers.
So it makes the live scoring meaningless. If you want to have it like in real life, fly the VirtualSoaring races. They score with IGC files and have the live scoring turned off. Only when everybody is finished can you determine what the "best start" in terms of scoring is.
