Well, exactly what happened is more than 10 percent of these young adults who tested positive for sexually transmitted infections (STIs) reported they’d not had sexual intercourse (defined as penile-vaginal sex) during the previous year, according to a 2011 study analysis published in the journal Pediatrics.

For the study, researchers interviewed almost 21,000 teen participants in 1994 about engaging in sexual intercourse and continued to survey them over time in different stages (or waves). In 2001 and 2002, scientists talked to about 15,000 of these teenagers again. Of this number, about 14,000 also provided urine samples researchers tested for three commons STIs—chlamydia, gonorrhea and trichomonas. The results? Out of all participants who tested positive for any of these STIs, about 6 percent reported they’d never once had sexual intercourse.

Obviously, there’s a disconnect. Why did young adults who reported never getting busy test positive for diseases that are usually transmitted through penile-vaginal sex (though all three STIs can be spread through oral and anal sex as well)?

Well, first researchers considered the possibility test results were inaccurate. Then, they suggested participants couldn’t recall they’d had sex because too much time had passed between the sexual act and when they reported the encounter. What’s more, the study showed no differences by gender, race, age or education for the young adults who said they’d had no sex but still got an STI.

What does this prove? According to scientists, just one thing: Self-reported behavior is not always a reliable indication of STI risk.

Jessica McDermott Sales, PhD, one of the study’s authors, says: “Given this and the high rates of [STIs] observed among young people, perhaps it’s ideal to screen all young people for prevalent [STIs] during medical visits.”