Torah Temimah cites Kerisos 28a.
Rabbi Shimon says: Sheep take precedence over goats everywhere. Might this be because they are preferred? Scripture says “and if he brings a sheep as his offering for a chatas” — teaching that the two are equal.
Diyuk of the optional “im keves” — equalizes sheep and goat for the ordinary individual’s chatas.
Although sheep generally precede goats in the order of preference, for the ordinary individual’s chatas the two species are of equal status. Either a female sheep or a female goat may be brought. The verse’s “and if” language prevents any inference that the sheep is preferred over the goat in this context.