We now know why.
Although Bills general manager Buddy Nix called Kolb a "strong candidate" to start at quarterback, a league source with knowledge of the deal tells ProFootballTalk.com that his contract with the Bills "screams backup."
At the time of the signing, Kolb's contract was advertised as a "maximum" of $12 million to $13 million over two years. In reality, Kolb is guaranteed just $1 million. The difference is tied up in incentives and bonuses that will kick in only if Kolb does indeed win out as the starter -- and plays well.
It's a matter of when -- not if -- the Bills draft a quarterback later this month. As long as Kolb outperforms Tarvaris Jackson in training camp, Kolb's roster spot should be safe. If he doesn't give the coaching staff a reason to keep him, though, it will be relatively painless for the Bills to cut bait.
Follow Chris Wesseling on Twitter @ChrisWesseling.