While most Amoraim were familiar with the mishnayot of the sedarim which formed the curriculum of Amoraim study, such familiarity was not universal. … Even when a relevant mishnah is cited, it is not always verbatim. …

The difference between the Bavli’s citation of mishnayot and those of Toseftan baraitot is that the text of the Mishnah has been transmitted along with that of the Bavli from the earliest times.

Yaakov Elman, Authority and Tradition: Toseftan Baraitot in Talmudic Babylonian (New York: The Michael Scharf Publication Trust of the Yeshiva University Press; Hoboken, NJ: Ktav Publishing House, 1994), 48.