sorority rankings

How do people decide if a sorority is considered top tier or not? In all the rankings I've seen Kappa, DG, and ChiO are always in the top and then ADPi and Zeta are usually bottom. I know looks actually count for rankings but how do some chapters end up so low compared to others? Zeta and KD don't seem that bad to me yet they're usually middle to bottom tier to everyone else.