Our Gemara records a machlokes between Rabbi Meir and the Rabbanan regarding the words and order of the viduy confession. Rabbi Meir follows the order of the verse used by Moshe, Avon, Pesha and Chata’a. The Rabbis cannot accept that because logically, the confession should go in order of severity: Chet (unintentional), Avon (intentional) and Pesha (Rebellious). If so, why indeed did Moshe use a different order? It was to ask God, let the great and even rebellious sins be considered as unintentional sins.
The verse quoted by Moshe was in regard to Thirteen Attributes of God’s Mercy. Tosafos (Rosh Hashanah 17a) enumerates what the Thirteen Attributes are in reference to the words in the verse and notes that “Chata’a”, that God forgives unintentional sin, as one of the Thirteen.
The Beis Halevi (Ki Tissa) asks a question on Tosafos based on our Gemara. According to the Chachamim, the word Chet in the verse is not referring to the actual transgression, but rather as an appeal to consider the other rebellious or intentional transgressions as unintentional. Since we rule according to the Chachamim, how can we say that Chata’a is one of the Thirteen Attributes? The Beis Halevi also raises another question on our Gemara. Why did Moshe just ask for sins to be treated as unintentional. Why did he not go all the way and ask that sins be treated as utterly forgiven?
The Bais Halevi answers both questions by offering a reinterpretation of how the Chachamim interpreted the verse. Moshe was NOT asking to consider the other rebellious or intentional transgressions as unintentional. Rather, Moshe was asking that grave rebellious sins be forgiven as easily as unintentional sins. This answer explains how Chata’a can still be referring to unintentional sin, even according to the Chachamim and be part of the Thirteen Attributes. It also explains that Moshe was asking for the best forgiveness, that the Jews be totally and easily forgiven as if their rebellion was merely unintentional.
I will add two points. It is possible to interpret the Gemara and the Chachamim as we originally did, that Moshe was indeed asking Hashem that the great and even rebellious sins be considered as unintentional sins. This itself could be understood as a unique attribute of mercy. And, as to why Moshe did not ask for total absolution, that the sins not be counted at all, this is not a question either. We find that Tzadikkim know how far they can push and what can be asked for, realistically. To ask for rebellion to be wiped clean as if no sin was done, was too much. Even Avraham, when asking for the people of Sodom to be spared, knew to stop at the threshold of ten righteous people. He does not dare ask God to spare Sodom if there was, say, nine righteous people.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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