At some places, the average is a little bumped up by outliers (i.e., the two >40-year-olds in my first-year class made our average higher than it would've been otherwise). But approximately 50% of medical students take at least one year off between undergrad and medical school.
Some people have really good reasons for taking time out. Others need a year for whatever reason and they scramble something together so that they have something they can tell medical school adcoms.
Lots of those students do research or some type of humanitarian job/volunteering (I had classmates who did AmeriCorps, Teach for America, and the Peace Corps). For others it's just a matter of logistics because applying to medical school is time consuming or because they want their last year to count in their application. Some people take a year out because they decided late that they wanted to go to medical school, so they can't take the MCAT until they've taken the pre-reqs, which doesn't happen until it's too late to apply for the next year. Some people have a less-than-great GPA and want to do really well during their senior year to bring it up before they apply. Others plan to (or need to) apply to 30 schools and don't feel like they can miss the time they'll need for interviews if they're in school.