Because networks put out press releases in advance stating that episodes will air, then change their schedules.
For example, as late as 11 Feb, ABC press released that the "The Beverly Goldberg Cookbook" episode would air 27 Feb (see picture). But then 27 Feb episode was switched last minute to "Our Perfect Strangers" and "... Cookbook" was bumped to 6 Mar. But then skipped 6 Mar and changed to 13 Mar. They did something very similar with Single Parents and Schooled. And that's out of thousands of episodes across our site.
Therefore, it's helpful when our members call our attention to the revisions so we can update the calendar.