it comes from Dryden's translation of Plutarch where he uses Sylla for the Greek Sulla, equating the upsilon for an English "y". I cannot recall that cognomen ever appearing in Latin as "Sylla."

Much the same is the explanation for why the King James Version has Cyrenius as governor instead of Quirinius. Since Greek (koine, at least) has no "Q", they had to make do with a phonetic spelling of KurhnioV which the KJV translators converted, as was the practice of the time, taking the kappa as a "C" and the upsilon as a "Y".