Why does this Pasuk refer to Gershon and Merari as "Nos'ei ha'Mishkan" and Pasuk 21 to Kehas, as "Nos'ei ha'Mikdash"?
Rashi (21): [Whereas Gershon and Merari carried the curtains and the planks,] Kehas carried the Aron and the other holy Kelim.
What was the procedure regarding the carrying?
Rashi: As soon as Degel Yehudah began to travel, Aharon and his sons took down the Paroches and covered the Aron 1 (and the other holy vessels, 2 and arranged the poles) 3 and the Bnei Gershon and Merari dismantled the Mishkan loaded the wagons and followed Degel Yehudah. 4 Whereas Kehas waited for Degel Reuven to travel, after which they picked up the holy Kelim and followed them. 5
Ramban (citing a Beraisa): Gershon and Merari traveled behind Machaneh Reuven and Kehas, behind Efrayim. 6