My DS5 does this too, including with books way below his level, and I do think it is related to reading quickly and scanning ahead. When my dad read with him over the summer, he kept testing DS by turning the page while DS was reading out loud to see how far ahead he was scanning. I was annoyed to see him testing my kid instead of just reading together, but then saw that it didn't phase DS at all, as he already had the next sentence committed to memory.