Just think of how many high schools, and how many colleges that are out there that have baseball programs. You add in club teams. And how many games each of those teams have. It's so many, and yet, there probably isn't enough people to scout them all.
Scouts are probably relying on information from a bunch of sources, including independent scouts, to make sure they catch the right one's with the limited amount of time you can see them. And if in that limited time, they say are sick or are overburdened at school, or just have a bad day, it could affect their placement quite a bit.
This was posted a while ago. http://scouts.baseballhall.org/ It's actually a really good look into what goes into scouting. And really how general it is at times.