Jamieson, Fausset and Brown on Numbers 29:7-11

7-11. ye shall have on the tenth day of this seventh month an holy convocation--This was the great day of atonement. Its institution, together with the observance to which that day was devoted, was described (Le 16:29, 30). But additional offerings seem to be noticed, namely, the large animal sacrifice for a general expiation, which was a sweet savor unto the Lord, and the sin offering to atone for the sins that mingled with that day's services. The prescriptions in this passage appear supplementary to the former statement in Leviticus.