It's hard to explain, much easier to see.

It has to do with the fact that the center of the lens for an HS10/20 is at the top/bottom of the screen (depending on whether you have a ceiling mount or not). So when you zoom, that edge of the screen does not move. The picture gets larger in all other directions. So say you have a 48" high screen and are showing a 2.35 AR film with the PJ zoomed in to your 16:9 size. The movie part of the image will be approx 36" tall, so you have 6" black bars at the top and bottom. Now zoom that out to max, the image zooms away from the fixed edge of the screen meaning the black bar is zoomed proportionally larger in that direction. Now if you have a 110" wide image, movie portion is nearly 48" high, but the black bar at the top is now bigger, and your 48" movie's top edge is 7" from the top, meaning 7" is also off the screen at the bottom. So you have to be able to shift that picture 7" or so upward. Using Zoom Player on my HTPC, I can do this easily, and it even remembers each movie and knows to shift it automatically. My only standalonde DVD player is a $40 cheapy that I have hooked to my old tube TV for watching DVDs I converted from VHS, so I just don't know whether a standalone player can do this or not. That's just one of the advantages of a HTPC and software based player, you have much more configurability/tweakability.
You could also get around this with a different projector that has "lens shift", I think. I'm just guessing here because the HS10 is my first PJ and it doesn't have that so I'm just speculating....