People might all get BG but it is possible, not likely but possible, to not ride that day if it’s a particularly bad day.
Looking at the chart, about a week ago they called back a total of 70 BG, and a week or so before that they called back 61. A bad day combined with BG running out in 1 minute and back ups starting relatively early could result in someone getting a back up group who was there early, and then not receiving a FP for the next day. The stars have to align in a bad way, but I think I would be a bit concerned if we wound up with a back up despite being there early, without knowing what kind of day we were going to end up with.