Why do people with GPS take the wrong turn on audax events ?
Because the GPS moves on to the next instruction as you approach the waypoint, and if you are careless when entering them and tired when reading them, it's possible to get confused. Fortunately, you can then check the track.
In the 3 audax I've done, routesheet use has been more common than GPS use, and none of the people I rode with were slowed down by not using GPS. However, on the Mildenhall 300, I saved the front bunch a length backtrack when we missed a bunch of turns. By the time anyone noticed, we were some distance from the route, but I was able to zoom out on my garmin and find a way back.