Since most demos don't come with cases, some people just stick them in the extra slot of another games' double disc case. Whenever you recieve a game with an extra disc, the seller probably either didn't notice, thought it was part of the cases' game or just didn't care.
I once got the Hyper Catalog 4 the same way with a double disc case game that I bought online.
I also once bought two Emerald Dragon cases off the street(literally). One had both ED books and the other had a manual from another RPG. The games inside were Ys III, Tengai Makyo II, Dragon Slayer II and a Chinese War Sim.
This was during the early days of the real internet (pre-eBay?) and I believe that that was how I got my first TM II. Back then, you couldn't easily find games like those (or much PCE stuff in general) for sale online.