There's a saying styles make fights.
Okami and Vitor were tailor made for Anderson, and Stann was tailor made for Chael. Vitor has frozen in the past and Okami is one of the more mentally weak guys in the sport. Okami had no chance in hell of beating Anderson in Brazil. Stann has some hellacious power in his hands but his ground game simply wasn't up to par with Chael's.
Chael's style is unlike Vitor and Okami's, we see this in the first Silva/Sonnen fight, Chael's ruthless and will come right at Anderson and will take no backwards steps or freeze in front of him. That could give Anderson problems, it could also hurt Chael leaving him open for counters and what not.
As for Silva's injury, he was clearly hurt but a broken rib? No way him or anyone else is stepping into a cage fight with a broken rib. People bring up the TRT and the broken rib and rightfully so, but on an even playing field Chael isn't exactly the ideal style match up for Anderson, Anderson can still win in many ways but he's not going to cruise like he did against Okami or any of the other guys he's fought.
THE PERFECT STORM
THE PERFECT RECORD
THE PERFECT FIGHTER