Ghrana closes the list of five Jnanendriyas, and its position at the end is not incidental. Samkhya's cumulative account of grossening adds one new tanmatra at each stage of the elemental sequence, sound first, then touch, then form, then taste, and gandha, smell, last, meaning that odour is treated as the newest and most material of the five basic qualities, present only once the sequence has proceeded all the way to prithvi, earth. A being capable of smelling is, on this account, registering the single quality that most depends on the physical density of what is being smelled, which is why gandha requires actual particulate contact in a way shabda, sound, does not.
Ghrana is derived, with the other four Jnanendriyas, from the sattvic aspect of ahamkara, and its restriction to gandha alone follows the same discipline as shrotra's restriction to shabda or chakshu's to rupa: one organ, one tanmatra, no overlap. The physical nose is, as with every other sense, only the gross location of a subtler faculty; the actual perceiving is done by ghrana, the tattva, which the classical system treats as ontologically distinct from the fleshy organ even while acknowledging their obvious practical connection.
Because ghrana is last in the sequence of both organs and tanmatras, it also marks the completion of the eleven-organ, five-tanmatra apparatus that ahamkara's sattvic and tamasic aspects between them produce, the point at which Samkhya's account of the perceptual and elemental machinery is fully specified and the system moves on, in its remaining tattvas, to the five gross elements themselves.