This sort of study is often used to investigate genetic relationships in a wide variety of things. It is assumed that, since identical twins share DNA and non-genetic twins do not, any variation betwen the two groups is a result of genetic differences.
In this particular study, the researchers concluded that "female orgasm is not all psycho-social as some suggest". What a revelation! That certainly wouldn't have been immediately obvious. I mean, really? I know that there are many out there who would like us to believe that orgasms are all in women's heads (that way, when women have trouble reaching orgasm it's all in their head as well), but it just seems so abundantly clear that it's not the case. The physical sexual parts of a woman's anatomy are no less real than a man's penis, they're just more poorly understood. To argue that orgasms are all in our heads when they are so clearly related to stimulation of specific parts of the body seems disingenuous.
That said, however, their conclusion that "between 34 and 45% of the variation in ability to orgasm can be explained by underlying genetic variation" is premature. Twin studies are used to determine correlations between genetics and behaviour, rather than causes. The researchers jump from noticing differences between the two groups that could be genetically related to concluding that the ability to orgasm is heritable and somehow provides evolutionary benefit. As always, I am wary of anything that smells like evolutionary biology or psychology.
The scientific world, it seems, will not rest until they find a reason for the female orgasm. They understand what the male orgasm is for. It is necessary for men to orgasm if they are to create offspring. The purpose of the female orgasm is less clear. Some research indicates that women are more likely to orgasm during periods of fertility, and some research indicates that sperm uptake is increased during female orgasm, but neither of these things has been consistently replicated. They're just theories presented in the hopes of explaining why it is that women should be allowed to enjoy themselves.
Another twin study found that when women try to reach orgasm through masturbation, about half their chance of success was genetically related. This study also found that during penetrative sex genes accounted for only 31% of their success. The article then indicates that "researchers don't know why the frequency of orgasm during masturbation has a higher genetic component".
I don't know about their genetic components, but I can certainly explain why there's more variation in success rates when partners are involved. Sex with other poeple is more complicated than sex with yourself, for a whole slew of reasons.
Most glaringly, we know ourselves better than we can ever expect someone else to know us. I know exactly what to do to get myself off. Sometimes I vary it, sometimes it doesn't work, but I know. I have never had a partner who knows what I like as well as I do, nor do I expect that will ever be the case. I live in my head. They don't.
Perhaps your partner knows you realy well, but is having an off day. Perhaps s/he's tired and isn't quite as into it as you. Perhaps you're irritated with them. Perhaps your partner knows you really well, and knows exactly how you like to be touched, and has been doing the same thing for three years and you now find it boring rather than erotic. Perhaps you're sleeping with someone you're really attracted to, but are still sort of nervous around and you can't relax enough to orgasm. There are so many reasons that variation exists in partnered sex that it's impossible to list them all.
A final word on things comes from another researcher who points out that while research into the genetics of orgasm is interesting, it is of limited use. Sure, she says, there may be a genetic component, but that doesn't really help us to understand anything. Genes could "influence the reproductive tract, and also the development of the hormone system, the nervous system, and the brain, all of which can influence orgasm". Even with a genetic component, then, orgasm could be all in our heads.
1. BBC News - Female orgasm is 'down to genes' - 07/06/2005
2. News in science - female orgasm is in the genes - 21/02/2005