Perhaps (s)he likes to self-identify as politically correct? There can be many, many explanations for the handles people choose. Be careful that you don't find yourself trying to make sense of a Rorschach Inkblot...
Are you saying that the person doing this has no conscious access to his or her own latent reasons? Or that this is just a random pattern like a Rorschach? I don't think it is a random pattern. There are far too many xx usernames here, relative to the other online communities I am a part of.
I'm saying that trying to surmise what someone is thinking from their choice of a two-character handle is like interpreting a Rorschach, which is not the same thing as saying that the characters themselves are randomly generated.
http://en.wikipedia.org/wiki/Rorschach_inkblot_test