I've heard the argument (elsewhere on the site) that because the DSi has exclusive games, it should be counted as a seperate system. There were a number of GBC exclusive games (I myself have about 8, compared with 12 or so GB games) and it was counted as part of the 120million GameBoy total. It was primarily the same hardware as the GB, with colour added. DSi is the same hardware as DSLite/DS but with flash memory, 2 cameras and no GBA slot.
In context, this isn't a vast step up over the capabilities of the DSLite, and the vast majority of releases will continue to be playable on all DS systems. The difference in DSi and DSLite/DS lies in the potential of future software, and not in any significant hardware step up. It will be counted as the third model of the DS family, and not just by VGChartz. Apologies for the longwinded post.