As far as I understood the myth of Medusa, she was punished by Athena (either for letting herself being raped by Poseidon or for succumbing to Poseidon's woes). Her sisters, Euryale and Stheno (all daughters of Phorcys and Ceto) stood with Medusa, and Athena transformed all of them in gorgons.
I'm not surprised about the punishment of the gods, neither to Medusa for her offenses nor to her sisters for staying with her (most Greek mythos depict similar punishments for something that is almost responsibility of the gods), but I'm surprised by the fact that Medusa is depicted as the only mortal of the Gorgons:
And again, Ceto bore to Phorcys the fair-cheeked Graiae, sisters grey from their birth: and both deathless gods and men who walk on earth call them Graiae, Pemphredo well-clad, and saffron-robed Enyo, and the Gorgons who dwell beyond glorious Ocean in the frontier land towards Night where are the clear-voiced Hesperides, Sthenno, and Euryale, and Medusa who suffered a woeful fate: she was mortal, but the two were undying and grew not old.
Source: Hes. Th. 275
Why this difference between the three of them? Is something that is attributable to Athena when transforming the sisters? Or for any reason they were different in spite of being daughters of the same father and mother? Is is just due to the definition of "immortal" (meaning from "not growing old" to "can't be killed or die")?