Our Gemara on Amud Beis notes that the two goats sacrifices brought on Shavuous serve as atonement for defiling of the Temple — by entering it while ritually impure, or for defiling its sacrificial foods by partaking of them while ritually impure.

The Gemara asks: once the blood of the first goat is sprinkled on the altar, thereby atoning for this defilement, for what sin is the second one sacrificed?

The Gemara answers the second goat is for any incident involving impurity that may have occurred between the sacrifice of that first goat and the sacrifice of this second goat.

Gur Aryeh (Vayikra 16:11) notes that this is the same reason why several of the Cohen Gadol’s sacrifices on Yom Kippur mention “he will atone for himself and his house.” (Ibid. 16:5, 11, 17). Although he receives atonement from the first offering, he continues to seek atonement from the other sacrifices, as perhaps he sinned in the interim.

I will add this may also be why the Shemoneh Esrei of Maariv after Yom Kippur has no change in the text. We ask for forgiveness as if it were a regular night of the year, despite having emerged from a day of prayer and forgiveness. Such is the nature of man: his propensity to sin is so strong that there is every reason to assume more atonement is necessary even the moment he achieves forgiveness. It’s like taking a brand-new car off the lot and immediately getting a scratch. But this is what happens.