Do we really see what we are supposed to see?

Today I didn't have to proctor the test, but since this is the students' first mock exam, I still went to school and went to each of the three classes to remind them of the dos and don'ts.  The first thing I said to them was the English test lasted only 100 minutes, unlike the Chinese test, which lasted 120 minutes.  However, when I turned and looked, I found on the blackboards of two classes the time schedules for the test were wrong.  The time for the English test was wrongly put as 9:20~11:20.  The right version should be 9:20~11:00.    At that moment, I was a little shocked, thinking to myself, "What if I hadn't come?"  Some students might finally find they didn't have time to write the composition because when they were about to start at 11:00, the bell would ring, which happened to my former students before. 

How could this have happened?   Had the students read carefully the schedule the school handed out to each class?  This reminded me of Helen Keller's "Three Days To See," in which she mentioned that people who can see very often have lazy eyes. 

I don't mean to put blame on any students here.  I am just wondering how well we human beings use our inherited ability.