Jamieson, Fausset and Brown on Numbers 7:10-11

10-11. the princes offered for dedicating of the altar, &c.--"Altar" is here used in the singular for the plural; for it is evident, from the kind of offerings, that the altars of burnt offering and incense are both referred to. This was not the first or proper dedication of those altars, which had been made by Moses and Aaron some time before [Le 8:11]. But it might be considered an additional "dedication"--those offerings being the first that were made for particular persons or tribes.