This is speculation, but many transgendered folks tend to present gender in an non-traditional way, as well as people in the midst of transitioning that may be

This is speculation, but many transgendered folks tend to present gender in an non-traditional way, as well as people in the midst of transitioning that may be "in between" presentations in a traditional sense. If you only train on cisgendered faces, you may only be training on sort of "default" gender presentations, facial structure, and the like, which may give a similar disadvantage.