Sometimes, while Bob is reading, a word, a sentence, or an idea on the page will trigger a memory, or a fantasy, or another idea, and quickly Bob's mind is lost in a chain-reaction of these distractions. This might be a minor irritation, but the text-tracking portion of Bob's brain keeps running, on auto-pilot, following words, sentences, paragraphs, even whole pages, while the rest of his brain is away on holiday.
All of a sudden Bob remembers that he was supposed to be reading, it's why he's there after all, and he wanted to do it. But now it takes energy to read backwards and identify the point where his consciousness took the wrong turn, off the page. Then it takes even more energy to re-read the subsequent text, text that his tracker thinks it has already read.
Poor Bob. Lucky for him, he's beginning to practise mindfulness, having been inspired by Mindfulness in Plain English, a short book describing the technique. Mindfulness is the habit of being aware of your immediate environment at all times, including most importantly being aware of what's going on inside your head.
Bob's able to catch himself sooner when a distraction arises, and is able to consciously decide whether to return to the page, or stop reading and pursue the distraction instead (and maybe even write a blog entry about it). Now, he's beginning to notice that he gets more read in less time, and the experience of reading is more satisfactory.
I'll let you know how it goes for him.