Scouts are not always right. Amatuer scouting can get very tricky, as one of CAnucks scouts once said in a recent interview. There are times, teams will make a decision based on projected outcome. for example they might "feel" that this player will be this kind of players in 7 years etc etc, or simply put it, they are just not correct. Barzal's draft year, he wasn't that impressive either, his stats, were great but not out of this world great, he was plagued with injuries and for a top pick he wasn't scoring much, not to mention he wasn't very big at the time either, things change.
but my point is, Amateur scouting is not easy.