The drilled holes in the Tailbone are sized as pilot holes, just big enough to allow the screw to go in but small enough to give good grip on the threads.
The user drilled holes in the deck are sized as through holes, these are big enough to allow the screw to fit easily in the hole and allow the head to snug up nicely to the surface of the deck when tightened. It is important that these user drilled through holes in the deck are the correct size.