Needs change quickly because of injuries, draft, FAs, and street pickups. Right now Moore, Clabo, and Winston are out there waiting to fill needs at G or RT. You might find a third round pick, a street guy, or even Parnell or Leary steps up and emerge as solid starters. What if you take Fluker and four guys you ranked higher all went later and had better careers? You'll look back and wonder "if only" you didn't have to reach for an OL back in 2013.
We've already felt that remorse for having been forced to take Tyron Smith in 2011 because he filled a desperate need when the team had JJ Watt high on their list. I'll bet they ignored all rankings and just took Smith. Looking back on it, the OL still sucked arse and JJ Watt has made the biggest impact of any defensive player drafted in over a decade. We were fortunate Smith didn't get Romo killed early in 2011, though it ended up not mattering because Free did that all by himself getting his ribs busted in SF.
Forcing the draft to fill needs not only usually costs you a better player, but trying to force a raw, borderline overweight player like Fluker into the starting lineup game 1 is just as risky as anything else. He might be ready, he might not be. If you draft him 7th overall, then he'd damned well better be.