Textbook Perplexity: Exploring vs. Understanding the Bible

I am trying to figure out why Stephen L. Harris has produced two very similar textbooks on the Bible, and what if anything makes one better than the other. As far as I can see at the moment, Exploring the Bible is conveniently divided into chapters that correspond closely to the number of times a class meets in a semester, but Understanding the Bible has color photos and diagrams.

If anyone has any light to shed on what the differences are between them apart from these superficial ones, I’d be interested to hear from you. I can’t figure out what accounts for their parallel existence – or the significant difference in price.