Think of it like this, you can buy the store brand ketchup or you can buy Heinz the top of the line. They are both ketchup but one is just a little better than the other.
That doesn't really explain the contradiction, and I noticed it too. The way Saban explained it, it certainly sounded like need was a factor that is built into the rating system itself. For example, you might rate a RB a "90" based purely on talent, then rate a QB an "89" and if you strictly follow the rating system, even if you have a RB you take the RB over the QB. On the outset, that's what it sounds like.
However, I think he might have been getting at player attributes. Everyone keeps trying to get a beat on who is a "Belichick type" player and who is a "Saban type" player. We know that we could see trends under Wannstedt and Spielman. They preferred smaller, faster type players, with certain common characteristics (one of them being speed). What Saban might be saying is that for different positions, you rate based on different characteristics. Or, he may actually be saying that you rate the same position based on different characteristics, depending on what kinds of characteristics you've already got at that position and at other positions.
For example. Miami consistently graded linebackers higher based on pure foot speed. But, what happens in a situation where you want a defense that has big dump trucks that can pile all over anything that approaches the line of scrimmage? Every time you went to the linebacker well, you kept going for the best linebacker you could find, which was usually a speedster. So, you don't have those role players on your defense. You're stuck with basically one kind of defense.
This still, however, clearly takes need into account in the ratings system, but at least it's not as simple as "Well, I think we need a RB more than a QB at this point, so we're going to take a RB no matter what." Furthermore, I'm not sure it would apply to every position. Saban will probably let Linehan decide much of the offense for himself where it concerns personnel, and if Linehan decides hey, I need speedy backs in my offense at all times, I need speedy WRs in my offense at all times, and I need big mutha effing linemen at all times, Saban may ask him a few pointed questions about certain situations and what would Linehan do to overcome the situation, just to prove it, but if Linehan presents a good argument, I do think you could see a consistent bias one way or the other on offense in certain groups especially the OL, and especially the WRs.