The PS3 and Vita versions both MASSIVELY outsold the XBLA version.
As someone pointed out earlier in the topic the XBLA version was released months after and people had already came to the conclusion the game wasn't worth the hype build up so sales and expectations were low.
I don't think people are adding 1+1 and making 2. To me it looks like the sales in general for RCR were very poor so having the Vita version 'massively' outsell the others isn't much of a task to begin with.