It's just like shift times for workers in manual labor jobs. You can't tell which of the people who are meeting up 8 hours a day is most productive based on if they where there for 8 hours and 12 minutes or 8 hours and 25 minutes. But you sure as hell can easily see that if someone is showing up 4-5 hours on a nine-to-five job, then something is wrong. And if that person tries to "fake it" but showing up for 8 hours but spends half the time slacking off, but coworkers and managers will call him on the bullshit.