Some of the pikemen are indeed were quivers with arrows on their back. Look at the 2 guys a bit right of the centre. There are 2 pikes around that can only belong to those 2. One of them has a quiver.

If it was an unit with both pikes and bows then all of the soldiers would have quivers so the only logical explanation is that it is indeed a fault and that some men are using the torso parts of archers.