My daily commute (when I rode a bike) was much shorter than a Tour de France stage. You have to consider that that's the average over more than 20 consecutive day-long legs.
I think you are underestimating the difference between maintaining an average speed during a whole day and maintaining that same speed for 20 minutes.
I think you are underestimating the difference between maintaining an average speed during a whole day and maintaining that same speed for 20 minutes.