From the inception of Bitcoin in 2008 BTC has been the generally accepted abbreviation for the Bitcoin currency. But to be represented internationally it has to follow international standards. This brings about XBT. This standard require that the first two letters represent the name of the country and the last letter the name of the currency. But Bitcoin is decentralized that's why this new name for BTC starts with an X just like XAU for gold and XAG for silver which are not controlled by a single country.
So don't be surprised when you see XBT somewhere representing bitcoin as it gains international recognition this will most likely be the symbol for bitcoin.
