The issue is that it doesn't make sense from a logical standpoint: if the NEXT trigger goes to the "next slide" chronologically, why doesn't the PREVIOUS trigger go to the "previous slide" chronologically as well? If I go forward with 1.1, 1.2, 1.3 and 1.4, the reverse should be possible.
However, if the user is too curious and uses the MENU to navigate from 1.1 to 1.4, then the PREVIOUS trigger goes back to 1.1 instead of 1.3, because it's actually the "last seen slide". If I flip a book to from page 1 to page 50, I should back to page 49, not page 1.
This is considerably annoying when you have to hardcode every PREVIOUS button on 20, 35 or even 50 slides.