פירוש על עבודה זרה 111:7

The Talmud now accepts this second baraita and reads the first baraita in its light. R. Akiva indeed holds that the liquid is not wine until it is put into the cask and there skimmed.

There are now three tannaitic opinions as to when it becomes wine: 1) Mishnah—when it goes into the vat; 2) Anonymous opinion in the baraita—when it goes into the vat and is skimmed; 3) R. Akiva—when it goes into the cask and is skimmed.

There are two resolutions to this difficulty. First of all, it may be that the entire comparison between tithes and yayin nesekh is unjustified. The rabbis were strict when it came to yayin nesekh—the liquid is subject to becoming yayin nesekh as soon as it goes down into the vat, even before it is skimmed. The dispute was only with regard to tithes.
But Rava did use a baraita about tithes in his understanding of the mishnah. Therefore, he must posit that there are indeed three opinions.