Stupid question

I bought a small carry-on duffel bag, and one of the large side openings has a zipper opening on top. What I don't get is that it also has a zipper opening on the bottom of that same pocket, and if the lower zipper is opened, everything would fall out. If I open both top and bottom zippers, I can stick my arm all the way through.

Why, oh why, does my bag have that lower zipper?