Agreed, Jean-Christophe, but I personally can see a functional purpose for BOTH scenarios.
For example, someone is sitting on slide 25 and might want to quickly review something in an earlier slide so they click on a menu item which jumps them maybe 20 slides earlier. Once they have finished reviewing that slide they might want to immediately return to where they were, but if they press the back button and they are taken only 1 slide behind where they are now, it may be hard for them to get back to where they came from (except by using another menu item again, maybe).
I'd like to have BOTH options available to me (and to the learner) so they can either press a back button (moves linearly back by 1 slide) or they can press a return/previous button to go back to where they last were.
Acrobat provides BOTH navigation methods and I use them both - regularly.