The discrepancy regarding Poison has been addressed more than once in interviews with former and current Capcom employees.
Final Fight developer and Arika founder Akira Nishitani stated he supposed the character could be male, but added it was up to the viewer to decide.
Street Fighter IV's producer Yoshinori Ono, when asked in an interview about the matter, stated "Let's set the record straight: in North America, Poison is officially a post-op trans***ual. But in Japan, she simply tucks her business away to look female." He later emphasized it again when asked about what female characters could be included in the game
Street Fighter IV, stating that it would be too confusing to include her due to the region-specific gender.