While sometimes it's content held back on the disk (which is a shady business practice) sometimes it's just the opposite, in that during the month or so of production (millions of disks don't just magically appear you know!) the developers are sitting around twiddling their fingers and decide (or are told to) do sometime more productive with their time, aka make dlc, which cannot possibly be included on the disk BECAUSE the disk content has already been finalized and is in the process of production.
Regardless, most of the time it's nothing even vaguely important, and only people who really love the game will ever buy it (and not even then).
Unfortunately it's hard to tell between developer love for their creation or corporate greed, so people often assume the latter, and get very angry over very very little.