I guess in this context I mean that the core gameplay and game mechanics aren't being altered from game to game. Folks could argue, but at least from my perspective, the changes from Halo 3 to Halo: Reach were minimal, and I'm not anticipating any reversal in this trend for Halo 4. Thus why I don't see any reason why developers would have an issue putting out a new game in a timely fashion.
The same could indeed be said of many games and their sequels, although many games do mix up their mechanics a bit. Super Mario Sunshire plays a good bit differently than Super Mario Galaxy, even though both are "Mario games". To be sure, titles like Call of Duty do seem to repeat the same formula, but with just enough innovation (e.g. "Spec Ops" mode) to keep things interesting for the majority of players. With regards to Halo, I was disappointed with "Firefight" mode myself, and even more disappointed when it wasn't enhanced in any meaningful way for Halo: Reach.