Nintendo use the same naming scheme (adding one or two letters as a suffix or a prefix) for revisions and successor systems. That's what causes the confusion. I can completely understand why many people thought that the 3DS was just a 3D version of the DS after the DS Lite, DSi, and DSi XL.
The DSi was also
a separate system; there just weren't very many DSi-only games released.
Really? Wow, I had no idea. I knew about the DSi store, but I had no idea that there were cartridges that only worked on the DSi.