The NFHS rule says "including" which implies there are also other ways for a pitch to be too low. NFHS could have phrased it much better if that was the only way for a pitch to be too low. I believe it is a clarification to include those pitches rather than an absolute standard for too low.The real difference in the NFHS definition of a WP is that little word “SHALL”. In most rule sets a ball the touches the dirt pretty much absolves the receiving player of an error if the ball isn’t handled and causes prolonged life for a batter or allows a runner to move up. The NFHS rule also does that if a pitch touches the dirt in front of the plate, but not if it touches the dirt anywhere else.
As you know, WP is charged against the pitcher and PB is charged to the catcher.In this particular case, the only real difference is, a WP has no effect on whether or not a run is earned, but a PB can.