I'm going to stick with my theory, here's why...
I'll agree that all turns can increase load factor. I don't agree that it doubles in a 60 degree turn unless you're in straight and level flight. If your airplane weighs 2000 pounds the wings must create 2000 pounds of lift in straight and level flight. In a 60 degree straight and level turn, the wings now must support the aircraft weight plus an additional 2000 pounds due to centrifugal force. The wings now must support 4000 pounds and load factor has doubled. If you take straight and level out of the equation, your wings don't have to support 4000 pounds, so load factor hasn't doubled. The load factor would be dependent on your rate of descent in a descending 60 degree banked turn.
I'll quote page 1-17 from the Pilot's Handbook of Aeronautical Knowledge. "A positive load occurs when back pressure is applied to the elevator, causing centifugal force to act in the same direction as the force of weight. A negative load occurs when forward pressure is applied to the elevator control, causing centrifufal force to act in a direction opposite to that of the force of weight. Curved flight producing a positive load is a result of increasing the angle of attack and consequently the lift. Increased lift always increases the positive load imposed upon the wings. However, the load is increased only at the time the angle of attack is increased. Once the angle of attack is established, the load remains constant."