Male circumcision among Kenyan population practiced among Christian and Muslim groups since long time ago

Ann, a Kenyan collegue was talking about the 'natural' male circumcision happening in the Kenyan population, among both of Christian and Muslim groups. She described them as religious versus non-religious groups in general. If I understood it well, she considered difference between the Christian and Muslim groups in their sexual behaviour, in the decision of how many partners to have etc... Ann assumes that HIV prevalence among Mulsim men group is much lower than the Christian men group although both are circumcised. Any sexual practice related reason or...? I tried to draw her discussion in the graph.