A fretboard that tapers in width is like a cone. Picture this: suppose you center the sanding block by the nut and draw two parallel lines going to the body end of the board. Easy enough to sand that parallel to the board's centerline. But you will have a long, narrow triangle outside those lines on each side. Each of those sections would have to be sanded exactly parallel to the centerline of the board, not parallel to the sides of the fretboard. It's doable in theory, difficult in practice.
I agree that a flat block won't work. That's fine for fret leveling, maybe Bill was thinking of that.
Gotcha, thanks Dave. Your example makes sense, but here's where my confusion comes in:
Question: An 8 degree radius sanding block (one that overlaps both edges of the fingerboard) is a constant radius, not tapered. Isn't the idea of a radiused sanding block to center the block on the centerline of the fingerboard and sand it down evenly?
Q Pt.2): Is it because the full-width sanding block can cover the entire fingerboard without moving off the centerline the trick to keeping the constant radius while the fingerboard width fans out?
Thanks for all the help here.