For many, it's an educated and informed guess taught by years of experience. And they get paid to do it.
Maybe it is that he really is seen as a rotational/roster player of 5th round value, like C. Landry feels. Landry has 25 receivers rated 4th round and higher above him.
A few things:
One: I try really hard to not get upset about things of which, I have no control.
Two: I trust the process of the Colts, meaning if the Colts sign AV then it's because they have tested him and are assured as can be that his issues last year were not because he's lost it, mentally or physically. I don't think they would sign him for sentimental reasons.