Jackie hit the nail on the head with her explanation (thanks, Jackie! :)). In conditions 1, 2, and 3, the patient is able to rely on somatosensory cues and does not lose their balance. In conditions 4, 5, and 6, somatosensory cues are altered. This requires increased dependence on visual (condition 4) and/or vestibular (conditions 4, 5, 6) cues for balance. If the patient had no impairment in the use of visual cues, they likely would not have fallen on condition 4.