Value lies in the eye of the beholder. Those outside of NFL draft war rooms must take into consideration that they don't have all the available information to completely assess a draft class. Thus, the board tends to fall differently than projected.
Those on the outside who cover the event don't have access to medical records or private interviews/workouts. They aren't taking into account scheme-specific preferences. They're not eliminating prospects from their board to shrink the number to under 150 draftable options.
All 32 teams have different draft boards. They're not working off the same homogenous list, hence why defensive tackle Kobie Turner turned out to be a third-round pick for the Los Angeles Rams and made a strong case for Defensive Rookie of the Year despite being 146th on last year's consensus mock draft database.