Why are there dots on "Aharon"?
Rashi: Because he was not counted together with the Levi'im. 1
Why does the Torah omit three hundred Levi'im from the total (Gershom, 7,500; K'has - 8,600; and Merari, 6,200), thereby enabling them to redeem the extra two hundred and seventy-three Bechoros?
Rashi (citing Bechoros, 5a): Because they were themselves firstborn, and it sufficed for them to release themselves from having to be redeemed.
Rashbam (on Pasuk 49): Because one firstborn cannot redeem another firstborn. 1
See Bechoros, 5a where the Gemara cites both of the reasons given by Rashi and the Rashbam.