The meat-offering was not mentioned before, because it was sufficiently understood from the law which required it, Numbers 15:3-5, but for greater assurance is here expressed.
Related Commentaries of Numbers 7:87
Numbers 7:87
87 All the oxen for the burnt offering were twelve bullocks, the rams twelve, the lambs of the first year twelve, with their meat offering: and the kids of the goats for sin offering twelve.