The projected ratings that you see are far from absolute. The higher your advanced scouting budget, the less variance you will see from the "true" projections. But even with the maximum AS budget of $20m, there will still be some variance.
Development of a prospect towards his true projections depends on three basic things: (1) good coaches; (2) playing time; and (3) timely promotions.
Things like injuries and demotions can hamper development.