Unlike most team sports, in MMA the best fighters don’t get to all compete against each other, much less compete against each other multiple times. Because of this some speculation is required to put together fighter rankings.
Prior to getting knocked out by Brett Rogers, Andrei Arlovski was top five in most heavyweight rankings. But who did he beat to get there? The only other top fighter he had beaten was Tim Sylvia, who subsequently avenged that loss. Arlovski never fought Barnett or Mir or Nogueira or Lesnar or Couture or Overeem. So we can only speculate as to how well he would do against them. But at one time he was the UFC heavyweight champion and since then he’s continued to accrue credible wins even if they weren’t against top ten fighters.
Arlovski’s loss to Fedor only means that he can’t be number one.
If Arlovski has beaten other top ten fighters, it would place a cap on how far down he could slide with his recent loss to Rogers. But since he hasn’t, and Rogers was not ranked when he beat Arlovski, the most logical decision is to take Arlovski off the top then all together.
And since Rogers just beat a ranked fighter he slides into the number ten spot.
Does this mean that Rogers would beat Carwin or Velasquez or (Antonio) Silva, or anyone else knocking on the gates? Absolutely not. But that’s just speculation.
Werdum, Nelson, and Rothwell, if not top-ten fighters, are all right on the bubble, I don't think wins over them count for nothing. Andrei's long history of credible wins and strong performances against top fighters is more than enough to keep him in the top ten, in my opinion.