Retcon idea: female skaven have a 1 in big number chance of being born incapable of breeding and those that are have an even slimmer chance of surviving to maturity. These extremely rare ratwomen are often hyperhoned into elite members of their clan, but rarely become lords. The even more rarer single female lord is such a godlike sniper that no mortal has ever seen her up close and thus continue the belief that there are no female skaven outside of broodmothers