The Gemara answers that the sciatic nerve is a distinct entity, and therefore it is different in that it is not subject to nullification.
וכן חתיכה של נבלה [וכו']: ותבטיל ברובא
§ The mishna states: And similarly, in the case of a piece of an animal carcass or a piece of non-kosher fish that was cooked with similar pieces of kosher meat or fish, when one identifies the forbidden piece and removes it, the rest of the meat or fish is forbidden only if the forbidden piece was large enough to impart flavor to the entire mixture. And if he does not identify and remove the forbidden piece, all the pieces are forbidden, due to the possibility that each piece one selects might be the forbidden piece. The Gemara challenges: Even if the piece of an animal carcass was not removed, let it be nullified by a simple majority, as the majority of the pieces are kosher.
הניחא למאן דאמר כל שדרכו לימנות שנינו אלא למ"ד את שדרכו לימנות שנינו מאי איכא למימר שאני חתיכה הואיל וראויה להתכבד בה לפני האורחים
The Gemara clarifies its challenge. This ruling works out well according to the one who said that we learned: Any item whose manner is to be counted, i.e., that is sometimes sold by unit rather than by weight or volume, is considered significant and therefore is not subject to nullification. But according to the one who said that we learned: It is only an item whose manner is exclusively to be counted, i.e., that is always sold by unit, that is considered significant, and is therefore not subject to nullification, what can be said? Since pieces of meat or fish are not always sold by unit, they should be subject to nullification. The Gemara answers: A piece of meat or fish is different, since it is suitable to give honor with it by placing it before guests. Therefore, due to its significance it is not subject to nullification.
וצריכא דאי אשמעינן גיד משום דבריה היא אבל חתיכה אימא לא
The Gemara adds: And it was necessary for the mishna to teach both the halakha that the sciatic nerve is not nullified and the halakha that a piece of non-kosher meat or fish is not nullified, as if the mishna had taught us only the case of a sciatic nerve, one might think that it is not nullified because it is a distinct entity, but in the case of a piece of non-kosher meat, say it is not significant, and it is subject to nullification.
ואי אשמעינן חתיכה הואיל וראויה להתכבד בה לפני האורחים אבל גיד אימא לא צריכא
And if the mishna had taught us the halakha only in the case of a piece of non-kosher meat or fish, one might think that it is not nullified since it is suitable to give honor with it by placing it before the guests. But in the case of a sciatic nerve, say it is not significant, and it is subject to nullification. Therefore it was necessary for the mishna to teach both cases.
דרש רבה בר בר חנה חתיכה של נבלה ושל דג טמא אינה אוסרת עד שתתן טעם ברוטב ובקיפה ובחתיכות
§ Rabba bar bar Ḥana taught: A piece of meat of an unslaughtered carcass or of a non-kosher species of fish that fell into a pot of kosher food does not render the contents of the pot forbidden unless it imparts flavor to the broth and to the deposits of food remaining in the pot and to the other pieces of food in the pot.
אוקי רב אמורא עליה ודרש כיון שנתן טעם בחתיכה חתיכה עצמה נעשת נבלה ואוסרת כל החתיכות כולן מפני שהן מינה
Rav disagreed with Rabba bar bar Ḥana and appointed a disseminator to stand before him and teach his statement to a wider audience, and he taught: Once the non-kosher meat or fish has imparted flavor to another piece in the pot, that second piece itself becomes non-kosher. And this second piece renders all the pieces of meat or fish in the pot forbidden, because they are of the same type; therefore, nullification does not apply.
אמר ליה רב ספרא לאביי מכדי רב כמאן אמרה לשמעתיה כרבי יהודה דאמר מין במינו לא בטיל מאי איריא כי נתן טעם אפילו כי לא נתן טעם נמי אמר ליה הכא במאי עסקינן בשקדם וסלקו
Rav Safra said to Abaye: Now, in accordance with whose opinion did Rav say his statement? It is in accordance with the opinion of Rabbi Yehuda, who says that a type of food mixed with food of its own type is not nullified. But if so, why does Rav state specifically that the non-kosher meat renders all the pieces forbidden only when it has imparted flavor to another piece? Even if it did not impart flavor to another piece it should render all the contents of the pot forbidden. Abaye said to him: Here we are dealing with a case where he cooked the non-kosher piece with one kosher piece and first removed the non-kosher piece before adding the other pieces. Consequently, the remaining pieces are forbidden only if the non-kosher piece imparted flavor to the piece it was cooked with.
Rava said an alternate answer to Rav Safra’s challenge of Rav’s statement: