Well, the closest to come to the number between 0 and 10,000 wins the processor, it's that simple.
You don't have to make an exact guess.
Drawing a member number out of a hat probably would be more thrilling, but how do we know the member we pick is paying attention to the email they signed up to the board with, or even wants the processor? Sure, we could draw another number, but if that one is the same way, it could take quite a few drawings to select a member who is eligible for it and who is available to receive it.
This way, the only people who enter the contest are the ones who actually want it.