Why do we say "vehu rachum yechaper avon" immediately after Yom Kippur, right at the moment when our sins have just been forgiven?
Perhaps this is because you cannot really ask for forgiveness until you realize the magnitude of what you have done. Once you reach this point, even if you have formal atonement, you will run to ask for forgiveness on your own initiative.
We fully reach this point only at the conclusion of Yom Kippur.
(R' Haim Druckman, Hoshana Rabba '06)