Well, you have to remember that the scouting reports are based off observations against their peers - fellow players between the ages of 15-20. So when a kid gets drafted, in most cases, he performed at least reasonably well against his peers.
The challenges comes when you try and translates what you're seeing into how that player before when the competition becomes bigger, stronger, faster, etc.
The reason why you seldom see noticeably bad scouting reports is because most guys who are drafted are amongst the top 1/3 of the players on their team or in their age group.
You also have to remember that any kind of scouting report is a delicate balance between evaluating talent, versus whether that players is likely to translate that talent. You could have a guy who isn't the most naturally skilled player on his team (though he's still skilled) but find that his game has a higher probability of translating to the NHL. Similarly, you could have an incredibly talented player whose game is not likely. It's tough to distinguish the next Marc Savard from the next Peter Sarno. Even people who have a solid track record have some complete misses.