I talked with a neigrabroador who is an anesthesiologist and presides over many spinal surgeries. He said that the patient is placed with the face in a pillow that has areas cut out for the brow bone, nose, ears and chin and they remain there during the course of surgery. Some people end up with bruising. Despite their best efforts at padding, patients sometimes end up with bruises, broken skin, etc.
I ended up with a femoral nerve injury in my right thigh after my first fusion, due to the way I was padded on the table.
Also surgeries are unpredictable. There is a ballpark time for each surgery so they know how to schedule the operating rooms, but the surgeon never knows for sure what he'll run into until he gets in there. Some times there is scar tissue that takes a long time to cut around or get through. Sometimes there is a bleeder that takes more time. ETC.
Did your father have other bruising over his body? I hope the remainder of his hospital stay will be more positive and that he'll have an uneventful recovery.