That is confusing... but my best guess is from wikipedia:
If originating superior to the optic chiasm, more commonly in a craniopharyngioma of the pituitary stalk, the visual field defect will first appear as bitemporal inferior quadrantanopia, if originating inferior to the optic chiasm the visual field defect will first appear as bitemporal superior quadrantanopia.
This implies to me that craniopharyngiomas arise for the stalk so they are supratentorial and other pituitary adenomas arise from the glandular tissue itself so they are infratentorial. Make sense?