The key to understanding Tosfos is to recognize that he argues on Rashi's explanation of the Gemara. Unlike Rashi, Tosfos understands that the Gemara's argument is regarding the Mitzvah of Melikah le'Chatchilah. Accordingly, when the Gemara proves that the Mishnah excludes the opinion of Machzir Davka as being le'Chatchilah (as the Mishnah's terminology of "Hamolek" according to Tosfos is le'Chatchilah, a point which the Maharam says Rashi takes issue with), it doesn't use the harsh term of Tiyuvta reserved for a mistake. It uses "Mistavra" to show that Machzir Davka, although always Kosher be'Dieved, is not le'Chatchilah.

All the best,

Yaakov Montrose