The Pokemon games are weird. Instead of being true sequels, it's more like each game is a piece of a bigger puzzle. Each one represents a different region of the world and they all fit together to make one really big and really engrossing world. It's like D/P doesn't really feel like it takes place after R/S/E. It's more like it exists for players to experience the Sinnoh region as opposed to the Hoenn region.

It's kind of neat that way. Like a mumorpuguh with it's expansion sets.