Question

Rabbi Michi, hello,
There is a dispute between Rav and Shmuel over whether, in monetary matters, we follow the majority or not. In a case where the buyer did not define precisely which ox he wanted and simply said "an ox," we follow the majority, and he can say that he meant an ox for plowing. By contrast, in a case where most people call a jug a jug and a barrel a barrel, while a minority call a jug a barrel and a barrel a jug, Tosafot say that in this case we do not follow the majority in monetary matters, and they maintain that even Rav, who holds that we do follow the majority in monetary matters, would agree that in the case of a jug and a barrel we do not follow the majority in monetary matters. I would appreciate it if the Rabbi could please explain the distinction between the two cases, as brought in the first Tosafot in the first chapter of "One Who Places the Jug".