Wood has to be split to be book matched. It's split along its thickness, and then when the "book" is "opened," the grain on each side has fibers running opposite to one another. This opposite grain direction can make even perfectly matched pieces appear to be unmatched to the untrained eye.
Also when the wood is carved, slight differences in the depth of the wood also create the appearance of differences. This is simply a natural consequence of a deeply carved top.
It's an old art taken from woodworking furniture. I personally love it. PRS occasionally has one-piece tops available, but I love the look of the book matching.