Countdown to High Noon in Real time

I've often read reviews where they wrote that Zinnemann was brilliant to film High Noon in real time. From the moment Cooper gets the telegram that Frank Miller has been paroled and looks at the wall clock(10:40 a.m.) to High Noon when the train whistle blows is 1 hour 20 minutes. Screen running time for the same period of time is exactly 60 minutes, 1 hour. So the reviewers are wrong, Zinnemann compressed 1 h 20 m into 60 minutes. I wish he had done exactly 1 hour 20 minutes screen time, it would have been unique.