The sole reason one half is Treifah is because of a nail was pierced in the stomach.
A butcher Schechted 7 cows. All seemed to be kosher. He sent 2 ½ cows to the shop
(Yatkeh) and the rest he kept at home. After the 2 ½ cows were sent away (Porush) he
discovered one stomach(unknown from which cow) was pierced (Treifah)
There are two important rules in רוב .
כל קבוע כמחצה על מחצה
At the place where Safek originated we don’t follow majority.
כל דפרוש מרובא פרוש
If the object left original place, you do follow majority. The two whole cows at the Yatkeh, (Porush) we follow majority (6 out 7 are kosher) and are kosher
The four whole cows at home, (Kavua) we don’t follow majority and are Treifah. The two Halves, we have a three way Machloke, if both are kosher or both are Treifah and the Pri Chadash who holds , the half at home (Kavua) is Treifah and the half at the Yatkeh is kosher.