For you people holding onto the idea that the game is 10-15 hours long because the developers say so: I can tell you from experience that number is purely an estimate and I can easily see how it's can be off.
Generally, when you're planning out a level (or chapters it seems in this game), you plan out how long you want the average player to take. It seems like they wanted each chapter to take 1 1/2 to 2 hours to complete and it could have originally taken that long. During the development it could have come up that the game was too hard or cuts in levels had to be made that seriously shortened the time. I imagine fighting got simplified (easier) and enemies were reduced for level optimization and that could more than likely account for the missing 4-6 hours.
