There are a lot of flights that are darn near impossible to overblock. I have been arriving 30-40 minutes early about 1/3 of my flights lately. For those, I am on time because it is impossible to overblock without drawing serious attention to yourself unless there is a ramp closure or the like. Also, on day 4, no one wants to overblock. That, combined with the people that are always on time, and we are about at the threshold for the bonus anyways.