The Gemara goes through many mathematical calculations to show how the churban came out on a motzei sheviit even though there are difficulties with that cheshbon.
Often, however, when the gemara deals with history it asks the question: "Moy dehava hava?" "What difference does it make, what ever occurred is what occured?" An answer is then provided following that question. Why doesn't the gemara pose that question here in our case?
Samuel Kosofsky, Hillcrest NY, USA
The Gemara generally asks "Mai d'Havah Havah" when the entire point of a statement is to figure out what happened, as in the Gemara in Yoma (5b) regarding, "How did he dress them?" However, in the Gemara here, this is not Rebbi Yosi's point. Rather, it was incidentally part of his proof that "Megalgalin Zechus l'Yom Zakai...." Once he brought up these facts as part of his proof, the Gemara analyzes how he knew that this was what happened.
All the best,