This is an example of what is called “inattention blindness” or “change blindness”. The idea is that people often miss large changes in their visual field. This has been shown in many experiments.
So what does this mean if you are designing a website or something on a computer screen? It means that you can’t assume that just because something is on the screen means that people see it. This is especially true when you refresh a screen and make one change on it. People may not realize they are even looking at a different screen. Remember, just because something happens in the visual field doesn’t mean that people are consciously aware of it.