Book publishing: typos and other dumb errors

When I published my first book -- a non-fiction story, God's Triangle -- it was edited by one person and beta-read by four other people. As far as I know, it contains no errors. However, my new thriller, The Mortal Maze, has been professionally edited and proofed at some expense by two people, been edited again (free of charge) by a third person and checked for errors by at least six beta readers. Yet when the book proofs came back from the printers earlier this week we still found a significant number of typos. How come? What's so different with this book? Is it because several of the readers told me they got completely carried away by the story, thus failing to note errors? Or could there be some other reason? 

One interesting suggestion is that proof readers be required the read a book twice -- once front to back, then from back to front. That way, it is said, few errors will be missed.

Anyway, The Mortal Maze is now available error free (I hope).