Here are a few theories introduced by the researchers in the study:

Women's genital responses are hidden from view and produce fewer "somatosensory cues." While men may get turned on by feeling themselves get erect, women do not. However, [studies have found that] even when women received feedback about their level of vaginal engorgement, correlations (between genital and subjective arousal) were low and statistically non significant. [In other words, being told we're getting turned on doesn't necessarily turn us on.]

Women may edit their self-report of feeling sexually aroused because of socially desirable responding. Positive affect directs attention to erotic stimuli, thereby increasing sexual response, whereas negative affect interferes in the processing of sexual cues, resulting in lower sexual response. Lower concordance among women may reflect their experience of negative affect while watching the conventional, commercially available erotica that is primarily produced for men.

Genital response to sexual stimuli may be an evolved self-protection mechanism. Female genital response is an automatic reflex that is elicited by sexual stimuli and produces vaginal lubrication, even if the woman does not subjectively feel sexually aroused...Female genital response entails increased genital vasocongestion, necessary for the production of vaginal lubrication, and can, in turn, reduce discomfort and the possibility of injury during vaginal penetration. Ancestral women who did not show an automatic vaginal response to sexual cues may have been more likely to experience injuries that resulted in illness, infertility, or even death subsequent to unexpected or unwanted vaginal penetration, and thus would be less likely to have passed on this trait to their offspring....Reports of women's genital response and orgasm during sexual assaults suggests that genital responses do occur in women under conditions of sexual threat.

That women can experience genital response during unwanted sex or when viewing depictions of sexual assault suggests that women's vasocongestion response is automatically initiated by exposure to sexual stimuli, whether or not these stimuli are preferred, and without subjective appraisal of these stimuli as sexually arousing or desired.

During processing of sexual stimuli, brain areas associated with emotional inhibition are activated among women. The anterior cingulate cortex (ACC) is a region of the brain where subjective responses to sexual stimuli are processed.  [The ACC appears to influence subjective responses but not genital responses, which helps explain the "upstairs/downstairs" disconnect. Incidentally, women's ACC is most active when we're ovulating and attracted to macho, high-testosterone men.  The ACC is activated when we're in conflict about something. Is the ACC also acting as a self-protection mechanism, warning us to proceed with caution?]

Bottom line: Physical arousal is not proof that a woman is really turned on. To really get a woman hot and bothered, you have to start from the top.