You need to post screen size and throw distance. I am pretty sure that your problem is this:
You are throwing a 4:3 image onto a 16:9 screen, with the image zoomed so that it fills to the sides. In doing that the bars would be outside the screen on the top and bottom.
That is pretty much the only explanation, since you have exchanged the projector and have the same problem. Nothing wrong with the projector, it is incorrectly set up. You may have the projector too close to the screen so that you can't fill the screen when in 16:9 mode. That is why you need to give us the screen size (height and width) and the throw distance.