Many fans still take the stars too seriously, as though they were the gospel. Many 4 stars are really 3 stars or 5 stars, many 3 stars are 4 star or 5 stars, many 5 stars are 4 stars. etc. If a top recruiting staff knows what they are looking for and have an eye for talent, if they take 3 stars at this point, they are likely better than available 4 or 5 stars.
Nick Saban's greatest"gift' is probably his ability to evaluate talent, thus his classes are usually better than their ranking. But the recruiting services know this and bump many of his players up quicker than they would others. Also, Saban is looking for specific types of players that fit what he wants. IMO, from 08-15 LSU had more athletically gifted teams than Bama, but Bama's fit Saban's style of play. Now that Bama has changed it's philosophy, Bama's teams are much more athletically gifted than before.
Bottom line is that though the rankings have some value, they are not close to being a precise measurement even at the class level and much less at the individual level.