I've seen a lot of DLC discussion the past few weeks, and I keep seeing members comment on on-disc DLC being identical to day-1 DLC. Even Pachter said it in one of his recent talks.
There's just one very distinct difference I'd like people to remember: On-Disc DLC is completed on the printed disc and was ready when the game went into printing. Day-1 DLC (or DLC within the first few weeks) is often created when the development team has finished making the game and has started finding down-time between the debugging, finalizing, printing of discs, and release date.
This is another reason why more games are having Day-1 patches. These developers have more time to work on patches and content that they don't have enough time to finish on the disc but are ready in time for the product's release date. Keep in mind that there are a number of weeks between when a game goes "Gold" and starts getting printed to when the release date happens.
