""OR reading, while tired and thinking I am reading one thing and when I go back to reread it...it's like where did this come from? I did not see that!.""
The actual cause of that phenomenon is the mis-understood word.
When reading along, when one reads over a word that is not understood as it is used in the text, it will cause a blank spot in one's memory just after the word not understood.
This was a phenomenal discovery on the subject of study.
Its why a subject can become confusing.
Its why a subject can seem difficult to learn.
Its why kids drop out of school.
Public schools don't yet understand this.
Be sure you stop and look up the words that your attention hangs up on.
Those are the ones that aren't fully understood, cause blank spots in the memory and are a barrier to understanding what your reading.
They also put you to sleep.
Ever heard of kids sleeping in class.
Its the mis-understood word.