Who are they having relations with, then?
The studies are just based on heterosexual intercourse, but it's just the same way the traditional studies show men having more partners than women, just reversed. The reason the numbers are never equal, is because they rarely calculate the raw average. The only point of calculating the raw average is to get an idea of whether or not the study is even sort of accurate or to compare the average numbers to an earlier data set. Instead, most are looking for what the number of sexual partners the *average* man or woman has.
Okay, and here's the TL;DR part that is probably getting way more scientific than you're looking for.
So, in a survey done by More* magazine, we see that 25% of women have had more than 10 partners, while only 20% of men report the same. The average woman has had 9 sexual partners and the average man has had 7 (sorry, can't find the original study which would show the percentages) telegraph.co.uk/news/newstopics/howaboutthat/3685314/Young-women-have-more-sexual-partners-than-men.html
* We'll get to why these surveys might actually be more accurate than more "traditional" surveys in a bit. The sample size was quite large (2,000). I cannot find out how they gathered participants, however, and that does make a huge difference. If it's not completely random, that can skew the results. But, the results do stem with other surveys given.
Here's an interesting read on sex surveys and the kind of answers given. Now, this study is looking for the raw average, but it's because they weren't trying to measure the average of sexual partners, but under what conditions people were most honest about their sexual deeds:
What's interesting is women who thought their answers would be read, reported an average of 2.6 partners, while the ones that theough they were hooked up to a lie detector reported an average of 4.4. Compare that to men who gave an average answer of 3.7 when they thought their answers would be read and an average answer of 4.0* when they thought they were hooked up to the lie detector.
*A .4 variation would be expected in a sample size this small. If it were a survey of say, 5,000, .4 would be too high.
Well, this is problematic when the most reliable studies have been traditionally thought to be face-to-face interviews.
To compound issues, men and women guesstimate differently. Men tend to just give a rough estimate, where women tend to tally the partners in their head, by name. This gives men a generally overinflated number, and women an either accurate or lower number (human memory is one of the most faulty things we have, afterall). ualberta.ca/~nrbrown/pubs/BrownSinclair1999.pdf
And that's just the basics, there's even more variables. The age at which you ask participants, relationship status, cultural and ethinc background, ect. And you have the bias of the people doing the survey to take into consideration. While they might not mean to skew the results, very subtle ways they word the question (even subconsciously) can affect the results they're given.
Another thing, when you're looking at surveys, you need to pay attention to the wording of the questions asked of the participants. Surveys that just ask "How many sexual partners have you had" or "how many people have you had sex with" are going to be inherently flawed since there's a huge difference of opinion on what counts as sex. A good study will actually indicate the sexual activity or activities that count as "sex" for the purpose of the study.
Ultimately though, when you look at good research, there's so many more myths than facts when it comes to sexuality (did you know, for example, that men in relationships with feminists report a happier sex life and get sex more often?). And a big part of this has been the decades of either flawed research, misinterpretation of the research by people that don't know how to read a study, and good old confirmation bias (we tend to ignore things that don't stem with our perception of the world).