Have you considered the possibility that tweet authors either masquerade as another gender, or otherwise confound your gender determination scheme? (Unsystematic) experience with various communities on Twitter tell me that both sockpuppeteering (running extra accounts to push an agenda) and personal safety (masquerading as, say, male to avoid female-directed verbal violence) tend to generate accounts with an opposite gender than the name might indicate.

How do you handle twitter accounts that do not give accurate, or even culturally existent, "real" names?