The University of Florida has the same turnout results as the Washington Post turnout figures. Not that I am a fan of the mainstream news media but for basic stats they record the same data for US presidential election turnout figures.
Not really sure why you've turned this into a clash of sources, we are discussing the same issue.
My question from the outset was very simple and was just that - a question based on simple maths.
Why do you think this is?
My guess is going to be because turnout is directly calculated from the votes and any number you get before that is just a guess based on extrapolation. So what you've done here is potentially taken the guess as truth and then ask why the guess doesn't match the truth. Which is something I've already stated.