Hai,
Quite some time ago, I think it was patch 3.2, not entirely sure, Blizzard added the Extend Raid lockout thingy.. Well, let's say Oculus is your least favorite heroic (and for most, it is). If you do it once, and extend the raid lockout forever, you will have less chance on getting that particular heroic.
And thus, you'll have way less chance on getting the heroic(s) you extended.
-Aldun