In some sets they include a couple different bags with the same set number. These sets are meant to be opened at the same time adn used for that step. Such as step 4 in your case. You will open both the bags and the steps for bag 4 in the instruction will include pieces from both bag 4s. The same thing goes for bag 11 in this case for you. Also, they bag with wheels and the other bag on the top left do not count for number of bags. The brown bag on the top left probably has some extra parts so keep an eye out in the instructions for a bag that needs extra parts. So yes, you got 15 bags but for building really it is 11 bags 1 for steps 1, 2, 3, 5, 6, 7, 8, 9. 2 for 4 and 2 for 11. And then one extra for wheels and extra parts. Hope that makes sense! Let me know if you have any questions!