Jamieson, Fausset and Brown on Leviticus 4:27-34

27-34. if any one of the common people sin through ignorance--In this case the expiatory offering appointed was a female kid, or a ewe-lamb without blemish; and the ceremonies were exactly the same as those observed in the case of the offending ruler [Le 4:22-26]. In these two latter instances, the blood of the sin offering was applied to the altar of burnt offering--the place where bloody sacrifices were appointed to be immolated. But the transgression of a high priest, or of the whole congregation, entailing a general taint on the ritual of the tabernacle, and vitiating its services, required a further expiation; and therefore, in these cases, the blood of the sin offering was applied to the altar of incense [Le 4:6, 17].