The chickens aren’t separated in the store; all the butchering and separating happens in the factory. The store gets boxes of legs, boxes of breasts, boxes of thighs, etc.
It seems plausible someone was very new and mistook the drumlet bin for the drumstick bin. I’d bet your order wasn’t the only mistake before someone noticed and corrected them.
It may be worth contacting the shop with your photo, because it will be obvious to them that is a drumlet. May be worth a free meal.