Saying Ulichaporas Pasha in Musaf of Rosh Chodesh
This question is a slightly different take on this one. I have posted a comment there and await to see if it needs to be removed from here.
There are three expressions for different types of sin which we use widely, פשע and חטא, עון . Sometimes חטא is used as a generic form but otherwise all three occur together. Except, that is, in the musaf of Rosh Chodesh where פשע is only mentioned in a leap year.
It is inconceivable that this form of sin can only be atoned during this restricted period, so why is it left out during normal years?
With פשע, there are now thirteen expressions in this part of the Amida. But this is surely no justification for leaving it out!