That's what baseball's dealing with right now. Surprises are cropping up while some hot properties going into the season are dropping. Most of the top 30 picks when the draft goes down are going to be guys who weren't on the top 30 list back in March.
This doesn't even get into the fact that, unlike football and basketball, teams are drafting players who won't even see the majors for years. You're trying to project talent three or four years down the road. You've got to be part part scientist, part psychic, part witch doctor to get the baseball draft on a consitent basic.