Shalom,

You are asking a very basic and good question. It touches the cornerstones of Torah sh'B'al Peh.

Indeed, the Rambam in the preface to the interpretation of the Mishnayos (and in other places too) wrote that there cannot be a dispute in the Halacha le'Moshe mi'Sinai. But we found in several issues disagreements not about the content of the Halacha itself, but whether a certain Halacha was learned from Sinai or from a verse. (See for example Chulin 126).

The Ramban, in his Hasagos of the Sefer ha'Mitzvos (Shoresh 1), disagrees with the Rambam, and believes that there may be a dispute in a Halacha le'Moshe mi'Sinai.

The Chavos Yair (192) wrote that the Tosfos in Hulin also disagrees with the Rambam's words.

Regarding your actual question, there are different approaches on how to explain the many disputes regarding the lessons, at least according to the Rambam. Some have written that these Halachos are among the Halachos that were forgotten during the days of Moshe Rabeinu's mourning, and such laws can be forgotten. Many Achronim raise the possibility that the basic idea of Shiurim are Halachos from Sinai, but their application and the decision to which matter applies which lesson is a message written to the Sages.

The Ritva here brings an interesting idea from Rabeinu Meir. He says that the reason Kolshehu is enough when it comes to Meizid, is because although the Shiur of a sin that involves eating is a ka'Zayis, that is when we are coming to test the amount one needs to eat in order to consider the act as eating. When a person eats an Issur be'Meizid, he is not charged for the eating, but rather for the rebellion against the Torah, so it is regarded as if he himself considers the smallest piece of this Issur as something important enough, at least in his eyes.

Kol Tuv.

Aharon Steiner