There's an interesting article in the most recent issue of Physics Today on gender inequalities in physics. The author argues that one of the contributing factors is that undergraduate physics problems tend to presuppose a base of knowledge that favours males. Examples that I remember considerered cars on racetracks, engines, struts, etc. The authors argued that a disproportionate number of female students would get hung up on basic questions like what is a strut and get lost even before they got involved in the physics of the question.
Whether this translates into a bias towards accepting lower credentialled female candidates or not, I have no idea.
I would not be surprised if there was data out there suggesting that good-looking, or fit, or outgoing and friendly people tended to be accepted over ugly people, or fat, or introverted people in spite of credentials. I suspect this is true everywhere because people tend to favour other people they enjoy being around.
There will be biases in any system. Graduate acceptance methods will not be perfect. When sexism is blatant, people generally have recourse options to correct for it. However, when it is subtle, it hurts everyone.