"We mirror our Top247 after the NFL Draft. There are 32 first-round picks every year, which is why we have 32 five-stars. So, if we award a prospect a fifth star, we believe that they have one of the best chances out of anyone in the cycle to eventually blossom into a Day 1 pick."
I could only find the football explanation quickly but my recollection is that the basketball rankings followed a similar reasoning projecting to the nba draft. The explanations continue to second round and other interpretations of the rankings. Clingan, Edey, Dickinson, were not 5* based on not being projected as future first round picks rather than purely a college impact projection