It isn't so hard to understand. Actually it's quite simple.
Some may not realize that you can't simply glance at your GPS to say that you're 'legal'. There are some places on the lake where there can be a significant current which can affect your boat.
In the first example, the current could be going 5 MPH or so, and your GPS could be indicating ground speed, not over the water speed. Which would the MP use (if the headway/steerage requirement was not in play)?