There is also this thing called wishful thinking even if there is a reasonable explanation to support a rumor though.
That kind of rumors were surfacing even before the 360 gained its huge userbase.
We have also seen them come true a good many times. VF5 was just confirmed, I'm sure the rumors of it to 360 were poo-pood by some but it happened.
MGS4, there is so much rumbling about that coming to 360 eventually I'd be surprised if it didn't. Almost seems more of an open secret to me.
Bioshock and Assisans Creed are newer games that the rumors also proved true (to either console). Heck even Oblivion..