They're ninja, there is no such thing as a fair fight. Putting that aside, Orochimaru killed the previous Kazekage, that shows that a Kage is not necciserally a powerlevel, but merely a badge of rank.But Gaara is a Kage... And Kages are stronger than any other ninjas in thier clan....
The only reason he lost was because that Akatsuki member was targeting the village... If it were a completely fair fight, or if Gaara didn't care about the village , he would have won..
Deirdra won because she had slipped a piece of Gaara's sand and mutated it to her own needs. The targeting of the village was merely a distraction.
You don't get the S-rank missing-nin label for robbing a candy store, you know. S-rank means Łber-powerfull. Yes, there may be varieties in power levels, but most of them are at least Sannin level.But you forget the point that not ALL of the Akatsuki are that powerful In any group, there are bound to be varying power levels.