[wideawake]

It's a regnal name.

For comparison, King George VI of Britain (the one from The King's Speech) was baptized Albert, but took the name George, after his father, when he was crowned.

The past two hundred or so Popes have taken regnal names, in order to honor someone they admired.

[Rocky Mountain Wild Turkey]

Anytime God wanted someone to go announce his plans to all the world, most of the time He changes their names ...

Abram --> Abraham
Simon --> Peter
Saul --> Paul

etc....

BTW, my confirmation name is Peter,