The uncommon impulse wave are not genuine impulse wave because they do not adhere to the normal Elliott wave rules.
For example, minor waves A and C of the zigzag pattern are uncommon impulse waves. The flat correction's C minor wave is an uncommon impulse wave.
Price segments AB and CD of an ABCD chart pattern are uncommon impulse waves or price-action.
Did you ask why they are uncommon impulse wave?
Well, it is because they are fulfilling the same role like a genuine Elliott impulse wave. They are directional and help to push the price-action from a point A to B. The main difference between the two types is that the genuine impulse waves have a distinctive structure and must not violate specific Elliott wave rules.
Most uncommon impulse waves are components of a corrective wave. However, not all uncommon impulse waves necessarily form a corrective wave.