High round picks are often a function of what the GM/owner wants, more than a function of who the college scouting department thinks is best. Think McGahee, Lynch, Whitner, Parrish, Hardy and on and on. Descretion over signing undrafted college free agents is more often given to the scouting department since these players are low risk/high reward propositions.
That said, the NFL is littered with quality undrafted players, so the ability to find them is not exactly hen's teeth...