I've come across a few resources like this one. In this case they claim:
unprotected vaginal intercourse with an HIV-positive man who is chronically infected and not on antiretroviral treatment (ART) carries an estimated 0.08% risk of infection for the woman per sexual act.
The most widely cited study of per-act anal-transmission risk was published in 1999. It found that amongst men in high-income countries, unprotected anal intercourse with an HIV-positive insertive partner carried an estimated 0.82% risk of infection for the receptive partner per sexual act,
Problem is I can find no mention in either of these two studies that they are referring to people who contracted HIV from their partner who they knew already had HIV. Is it really that hard to contract HIV from someone who is already infected? Or are these AIDS resources just mistaken in their claims?