A sin-offering for the people, as it is expressed here Leviticus 9:15, for whose sin a young bullock was required, Leviticus 4:14; but that was for some particular sin, but this was more general and indefinite for all their sins. Besides, there being an eye here had 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;