Extra pieces in 76112 Batmobile
They are in a perforated bag and the entire contents of the bag is listed. You only need some of them in this step and in a few steps later on. I think such bag is called an "accessory" bag and it is common to have the instructions like this for such bag. The first set that comes to mind that did the same was the
Big Ben 10253 where on step 269 on page 187 the entire contents of the bag with minifig tools is listed while you only need 4 times 2 pieces to build the clock arrows.