because there is one hiv+ and one hiv- doesn't mean the chances are 50% for aquiring hiv. if they have sex a hundred times they may never get it or they could have sex once and aquire it. there are so many factors on the reason it is transmitted. Was there long exposure, did one of them have a high viral load, did the sex cause bleeding, did he or she have cut that became exposed to any body fluids. For those reasons and many others, that is why there are many studies and as a result you get the statistics. 1/2000 for vaginal penetration with hiv infected person, 1/10000 for oral, 1/200 for anal and so on. you can aquire it the first time or maybe never if you are lucky.
There are several assumptions for that, what I think. It is an established fact, based on experimental data that a number of "poz-neg" couples are living happily without "neg" partner being infected. How I see this,
1. If female is "poz", then probability of male getting infection is much lower, even in case of an "unprotected" episode.
2. The couple, (knowing their status well), definitely more careful and takes all the precautions, like never penetrating sex unprotected, always using condoms, and doing oral and mutual masturbation. These are the proven techniques to keep onself away from this infection. Successful proceedings of such relationship enhance confidence on the proven protection techniques.