PixelBoy: In most cases this can be traced to stretch goals which got funded.
Right. A lot of projects add more content and even more mechanics, which will always push back the original goal (which assumes just the basic goal getting funded).
Another reason which happens more for very small teams (or individuals) is life coming in the way. Babies, job changes, illnesses, members having to travel for extended periods, all these things can have a significant effect on development.
Then again there's the feedback. One of the things Kickstarter is good for is getting feedback, and this can result in more fixes and changes. This can make the released game stronger, but it's usually not taken into account in the developer's vision of the game.