From the mentioned study: "The national rape-related pregnancy rate is 5.0% per rape among victims of reproductive age (aged 12 to 45);"
If you sample 1000 randomly chosen women of reproductive age, you would expect some proportion of them to be using some form of contraception such as an iud, oral contraceptive, slow release implant or similar which would block contraception. Some of them will probably be pregnant already. None of those women is likely to conceive as a result of being raped, and of the remainder, it is unlikely that on any given day more than 25% would be at a point in their menstrual cycle to be fertile.
I don't have all the numbers to hand, but I suspect that if you remove all the "can't get pregnants" from the pool of potential rape victims, you may find that the 5% of who fall pregnant is a much higher % of those who could potentially get pregnant from being raped.
How many couples do we hear about who try for 6 or more months before they succesfully conceive? That's less than a 17% chance of contraception, assuming all other possible negative factors are deliberately eliminated.
If only 1 in 4 rape victims is in a fertile window in her monthly cycle, then 5% of all rape victims of reproductive age suggests 20% of those who could potentially conceive are ending up pregnant as a result of the rape, and if other negative factors are taken into account, I suspect that the actual rate of conception amongst rape victims who could potentially conceive is even higher.