This source provides some further enlightening information:
"Three hundred thirty-two MTF transgender persons (112 African American, 110 Latina, and 110 API) completed the survey. ... The 3 most common self-identified genders were female (36%), preoperative transgender (31%), and preoperative transsexual (21%). Few had undergone sexual reassignment surgery; among them, 73% identified as female and 24% as postoperative transsexual. A majority identified as heterosexual."
This points out an interesting thing.  The large majority of the transgender individuals in the survey were pre-operative.  Since the only receptive intercourse available to them is oral, or anal, they are more likely to engage in URAS.  Honestly, I like the idea of "URAS", it should be: "People engaging in URAS" as the risk category... and we could avoid this whole "who's male who's female" idiocy.