This is a topic that has been discussed a great deal in the past with counselors, rape victims and other online forums. It is however still a taboo topic for Survivors to admit. It is usually thought of as not being PC or polite and people start getting very defensive about it. Well, exploring and trying to find out the truth is not helped by closing one's eyes to things we rather not believe.I think 5 percent would be remarkable. Around 20 percent seems absolutely astonishing to me, considering that it is estimated that only 30 percent of women climax via intercourse alone. Now, I very much doubt we'll ever get any reliable scientific data on the phenomenon, although I suppose it is remotely possible, given the current appreciation for EL James's literary adventures combined with the potential for forcibly administered pleasure, that a number of women might agree to be raped in the interest of scientific progress. It seems even less likely that rapists just happen to regularly target the minority of women who are capable of intercourse-only climax, even though that indicates some women must be more likely to have an orgasm from rape than from consensual sex.
I think that the answer to the question as to why do some females orgasm during a sexual assault or rape is that all of the various explanations have some validity, but none of them tell the entire story in the case of every individual victim of the crime. First of all, orgasms in women being raped are not frequent, but they are not uncommon either. In the study you cite, about 5% to 21% of women interviewed in the studies surveyed reported having an orgasm when they were raped. Researchers have hypothesized that the actual figure is probably a bit higher in reality due to victims being understandably embarrassed both by the rape and by having achieved an orgasm during unwanted, forced sexual relations. Around 20% seems to be a likely, real-word figure.
If that is true, this would have some astonishing implications for intersexual relations as well as the apparent reliability of the average woman's capacity for understanding the complicated intricacies of her own sexuality. It might also explain the otherwise inexplicable popularity of certain SF writers among the female SF/F readership.
The fascinating thing, of course, is the way that those considering the phenomenon are resolutely to avoid the obvious, which is that women respond in a sexually positive manner to male dominance, including violent male dominance. The idea that some rapists must engage in foreplay only demonstrates the intellectual panic aroused by the statistical observations. That women would respond positively to extreme sexual dominance should be no surprise, as it is very much in line with what Game Theory would logically suggest, and yet it is due to the fact that even the most experienced Game theoretician is steeped in decades of anti-scientific equalitarian propaganda.
Sexual response is a continuum, not a circle. So, it makes sense that the more extreme the input, the more extreme the output. Now, for the benefit of the cognitively challenged and the terminally offended, I will trouble to point out that no amount of orgasms can be used to justify rape anymore than an instinctive mouth-watering response to chocolate justifies force-feeding someone hot fudge sundaes, but it is both gutless and pointless to play ostrich and pretend that the reported phenomenon simply does not happen.