VGChartz has a bad reputation regarding sales numbers, because if they don't have real numbers they're instead doing an estimate. So numbers from them should probably be taken with a grain of salt.
According to Wikipedia, EA canceled the sequel, so I guess the game didn't sell a lot, but other things could be a factor as well.
For the Ron Gilbert game, they have said that it will probably make adventure fans happy, without confirming anything. So I guess it could be a "pure" adventure game or just have elements from adventure games
