Remember that 50% of all AS turned to the Dark but there were only about 30 or so at the same level as the 13 survivors ... at least from Moghedien's POV we learn only 29 individuals were ever given access to the TP ... and if she had not been one of them we would have heard her whine about the injustice of it all ... couple that with the fact that even Mesaana's deeds were written about and you have to figure these 13 were all at the very tip-top of the command chain for the Dark during the WOP.

I see 20% as a very large strength span especially with an even larger skill gap in there. To be honest, Graendal or Semirhage could have been weaker than Moghedien and still would have ended up ahead of her in the command chain because of their much greater skill levels. With a 20% strength disadvantage she would never have risen so high in the ranks no matter who she killed ... if she was that much weaker one of the others would have swatted her like a fly long before she became a threat. Back in the WOP they would have been able to openly move against each other because the DO had so many followers that he encouraged them to in fight "only the strongest serve the Great Lord" and all that.

The top 6 all had specific skills that made them stand out and when coupled with their very high strength they found themselves at the top, because there were so few that could stand against great skill AND great strength. IMO the other survivors were the next rung down from those 6 as they mostly had governance or military leadership roles during the WOP.