All projections that support draft decisions have a certain amount of optimism built into them: assumptions that the player will stay healthy, stay out of trouble, get stronger, improve his skills, and increase his knowledge of the game.
When a high school player is drafted, the pros have an idea how much better they expect him to get over the next several years.
In order to get drafted in the same position after college, his post-high school progress must track along with the progress projected back when he was first drafted so as to support projections for further improvement.
When a high school player who is drafted decides to go to college in hopes of being drafted in a higher position in a future draft, he is making a bet that his development will exceed the pro team's optimistic expectations when they drafted him and offered a bonus.
Implicit in that bet are the assumptions that he has a better grasp of his pro potential than the pros do and that he is able to realize that potential without their help.
It's seldom a good bet because those are seldom sound assumptions.