A sin-offering For the people, for whose sin a young bullock was required, Leviticus 4:15; but that was for some particular sin; this was more general for all their sins. Besides, there being an eye here to the priest's consecration and entrance into his office, it is no wonder if there be some difference in these sacrifices from those before prescribed.
Related Commentaries of Leviticus 9:3
Leviticus 9:3
3 And unto the children of Israel thou shalt speak, saying, Take ye a kid of the goats for a sin offering; and a calf and a lamb, both of the first year, without blemish, for a burnt offering;