What sometimes happens is that developers will create a game and then, very close to the end, some portions will be hacksawed out and turned into dlc. If done poorly, this can be very obvious and can ruin the game - the last Deus Ex game seemed to suffer from this.
This not standard practice. DLC plans will be specified at a high level early in production but will only solidify as the game gets closer to ship. There is a gap, which differs for different disciplines, between finishing work for the shipping build and its actual release. DLC is a great opportunity for the team to continue working at peak production on new content.
I've never seen content held back or cut out of a main release explicitly for the purpose of including in DLC. What sometimes happens is that features or half-finished content that the team decided to cut from the game is revisited for DLC. From my perspective as a game developer, modern games are never finished but at some point you have to start shipping things. DLC is an opportunity to explore ideas that would have otherwise been left on the cutting room floor, and provide some stability for large teams as they ramp down from full production.
