Tosfos D"H O Shelo, I had what I think is the same question:
On daf 3b Rabi Yonatan's aproach, that sanhedrin's 100% unanimity in p'sak is required for the par he'elam davar, is itosav (shlugged up), and the pasuk is used instead to create a 100% quorum requirement. Now on 4b Rabi Yonatan's approach seems to be assumed! The masores hashas points to a lechem mishna which seems to ask the same question. Does anyone answer it?
Marc Diamond, Providence, RI USA
Some Mefarshim (Keren Orah, Chok Nasan 3b) answer that R. Yonasan's approach was that every person is required to agree, even if there are one hundred people that sit down to judge. The Mishna, however, understands that only seventy-one need to be unanimous. The others do not have to agree. [The Maharsha and others give a different answer which I did not quote as the answer is deemed difficult by the Lechem Mishnah in Hilchos Shgagos 13:1.]
All the best,
Yaakov Montrose