The sphinx looks to me like it was built once, possibly as a complete lion (head and all)
Then it got its bottom third worn away by serious water erosion, then got buried up to its neck.
Then the face that remained was eroded away some more. So much that a pharaoh decided he would carve his face onto what remained sticking up out of the ground, possibly never knowing what was buried.
Makes more sense than what Hawass is babbling!