Part of a GM's job is to assess who's overvalued/undervalued, not just rely on journalist rankings. Not only did Rizzo get ripped off, he tried to cover his butt by giving Keibert that ridiculous contract extension.
Beyond overvalued/undervalued, there are lots of prospects who are top prospects because they have a high probability of being slightly above average. Part of the GMs job is weighing those guys vs boom or bust type guys.
