Men do not usually assume primary childcare responsibility, women do. This happens in their 20s-mid 30s.

This necessitates women either reducing their effort in grad-school or taking time off, whilst their career suffers. Of course some women do not have children, hence the comparison is useless, but in general society assumes women will have children and for a time be less productive.

Hence women get rated as less able to do work.

Political correctness is bullshit.