How did John's leg get hurt?

There are two references to the way in which Long John Silver lost his leg. The first is what Silver tells Mr. Trelawney. The second is what really happened while Silver was serving under Flint.... a pirate.

Long John Silver, he is called, and has lost a leg; but that I regarded as a recommendation, since he lost it in his country's service, under the immortal Hawke.

O, not I," said Silver. "Flint was cap'n; I was quartermaster, along of my timber leg. The same broadside I lost my leg, old Pew lost his deadlights. It was a master surgeon, him that ampytated me—out of college and all—Latin by the bucket, and what not; but he was hanged like a dog, and sun-dried like the rest, at Corso Castle. That was Roberts' men, that was, and comed of changing names to their ships—Royal Fortune and so on. Now, what a ship was christened, so let her stay, I says. So it was with the Cassandra, as brought us all safe home from Malabar, after England took the viceroy of the Indies; so it was with the old Walrus, Flint's old ship, as I've seen amuck with the red blood and fit to sink with gold."