expected female traits
If, from an early age, society gets skewed in what is expected from a man and a woman, then it is also expected that this inherent biases percolate all the way until they impact the female careers. For example, students judge female teachers differently than they judge male teachers1. This may be due to the fact that from an early age both boys and girls are exposed to what is expected from either gender (aggressiveness, compassion, caring, etc.)
The problem is when these biases surface in the form of evaluation questionnaires than then are used to promote one or another person. If the underlying data is skewed because the source is biased, then it only enhances the myth of the double blind.
See also: brilliance bias