Commentary for Avodah Zarah 149:2

There are two layers of disputes here as to how many processes of rinsing and washing with ash are required for a wet winepress or a dry winepress (the former requires less because it is already moist). The first dispute is between Rav and Shmuel and the second dispute is between the schools of Sura and Pumbedita as to what the parameters of Rav and Shmuel’s dispute was. The Talmud remarks that the latter version is not really a dispute because Rav does not count the final rinsing whereas Shmuel does. [I should note that there are other ways of explaining the last comment here].