Even with $20M in ADV scouting, some projections are going to be too optimistic and some too pessimistic. At 0 ADV scouting, I've seen players blow through the projected ratings in half a season and I've seen players with 39 CUR/98 PRO that were complete and utter trash.
The most significant things in determining projected ratings are CURRENT RATINGS, AGE and YEARS PRO. The fewer years pro, the more chance of ratings development. (i.e. recent 18 year old draftee, with one pro season develops more rapidly than any 4-year pro who is 26 and in AAA or the ML) And the better the CURRENT RATINGS, the better the chance the player is going to develop into a major leaguer.
Projected ratings are mirages - they might be real or they might not be. I cannot stress how important CURRENT ratings are. Those are dependable. If they are borderline ML quality at age 18 and the guy has decent makeup, coaching and health...he's probably gonna be pretty good.