The issue will never be position, and no a compass can't override 10 digit grid - they're telling you two different things. One is telling you position, one is direction.
It's when you're standing in one place, pull out your phone or your GPS, and use it to get a heading to your next waypoint or destination (Imagine trying to find your way somewhere in the dark and you stop to find out which direction you need to go). You put the phone / GPS away, and head off in the direction it gave you. A half hour or however long later, you pull out your GPS and the position is showing you that you headed off in the wrong direction because the direction arrow / compass was incorrect.
I can get around it with my Marsupial, and I love the harness otherwise, but I'm thinking about replacing it for that reason.