ESPN rankings take into consideration three main criteria -- productivity to date, projected college impact and long-term NBA upside.
Ideally, players atop the rankings project out in all three areas. Sometimes though you have a prospect such as Nassir Little, whose immediate college impact didn't measure up to his long-term NBA upside, or conversely, Gary Trent Jr., who was an immediate key for Duke but has struggled to gain the same traction in the NBA.
When it comes to the 2020 class, the contrast between college projection and NBA upside might not be as significant as the recent 2019 class, but there are still plenty of prospects who fall in only one of the two categories.