Look at the numerical distributions, we're exposed to believe that woman on average have had something like3 or 4 partners, while men on average have 6 or 7. Now pause for a moment. If men and women are having sex with each other, which they generally do, why wouldn't their averages match?
Clearly, unless there's a hugely unreported invisible underbelly of men having homosexual relationships with other men, or there's enough turbo sluts having sex with hundreds of guys in order to throw the numbers off by that much, something's wrong with this study.
Of course, it's all data collected by self reporting, as dimly pointed out, so (believe it or not) the most likely answer is that people lie about this sort of stuff.
My guess would be that women don't count things like blow jobs, one night stands, etc. as "sex", while men do.
That being said, all of this varies socially, economically, and geographically. I'd be willing to bet the average woman living in a city has had around 20 sexual partners in her life, while the ones living in small towns have 2 or 3 before getting married.
If you're trying to understand the world around you, well uchicago isn't sampling the world around you, they're taking thousands of people from whatever communities they use for these studies, and assuming that applies everywhere, across geographical distances as well as generational gaps. It doesn't.
Depressing as it is, I'm pretty sure /r9k/ has a better understanding of modern sexual practices than most of these studies. I really doubt the average woman in 2017 will only have 3 sexual partners in her entire life