On Rosh Chodesh, during the Musaf Amidah, we ask God to grant us several blessings for the new month. Among them are goodness, blessings, sustenance, etc. The last of the requests are for (using the Artscroll translation) pardon of sin and forgiveness of iniquity (m'chilas chait v'l'slichas avon). In addition, during a leap year, u'l'chaparas pesha (atonement of willful sin) is added as well.
My question, very simply, is this: Why do we only ask for chaparas pesha during leap years. Do we not need it during all other years?
Anyone know of any reasons?