B "reading"

So, I've been trying to capture this on video because it is so freaky weird. Several months ago, B picked up this book and "read" it word for word. As far as I know, it's not a book that had ever been read to him. Or if it had, it couldn't have been many times since no one remembered it. So imagine my shock that day. I'm still not sure what to make of it. It is only with this book so maybe he just has this one memorized somehow? Regardless, it is fascinating. This video isn't his clearest enunciation and he skips some of it. I have seen him literally say every word on every page but I can never seem to capture the best examples on video (of either of my kids). Still, if you try to follow along, you should be able to see that he is saying each word.  I have no explanation.