Remember that Sauron's Tier List is based on very high elo play. Meaning that when these champions are being used, they are being used by people who really, really know how to play.
Garen is a good example. If you go against a Garen that really knows how to play well, he can walk all over you. When you play against someone who knows how to kite, Garen gets walked on. I feel that this is applicable to many things...
Urgot and Nunu are considered very powerful bot laners, but if you don't know how to use them and how to take advantage of their strengths, no one would really see why these champions are powerful. Heck, Nunu was no where near cheesecake tier bot until someone really got to learn how to play him bot lane and people realized that a good Nunu can counter almost anything.
Although, with the Kassadin thing...we kind of don't tell people how to play Kass because we don't want to encourage people to use him...