FF XIII was crap, people bought because of the hype and it was the first next gen FF.
We can see XIII-2 that it decrease in all 3 region especially Japan. X and X-2 had a much better attach rate than 13 and 13-2.
Your statement is only valid for 13-2 because they compromised the game by fitting it all in 1 dvd.
13 had 3 dvds, we can see xiii-2 was clearly made to accommodate the 360.