I remember when Mega Man X came out a friend of mine thought it was "Mega Man Ten". Even though at that point Mega Man had only gone to 6, since the game took place in the future, he figured it was a clever marketing gimmick like this was the tenth Mega Man game from the future. Then you get something like Final Fantasy X-2 which is confusing in the opposite way.
In retrospect they should have called FF X-2, FF XI. The real FF XI is an MMO which for all intents and purposes is unrelated to the main FF series. They should have called that game FF Online and made it it's own spin-off series. And here we are again with Final Fantasy XIV being another MMO and we get FF XIII-2 which is effectively the "true" 14th FF game. Are they doing this on purpose is that just a major fluke? It's almost like someone at Square Enix does not want MMO FFs being included in the main series and has come up with this "sequel to a sequel" strategy to keep the chain of single player FFs unbroken.