That doesn’t mean 1-in-5 bring parents. It means 1-in-5 interviewers have seen someone bring in a parent. If an interviewer has 500 interviews and 1 brings in a parent, that interviewer is one of the five who has seen a parent at an interview. Even though it was only 1 in 500 interviews for them, they’re still 1-in-5 interviewers.
Hell, it could even be the same fucking parent at every interview, if it’s a small enough industry. Maybe that same college grad applied to all of the local jobs in the industry (because of course they did; it’s what they studied for) and so all the interviewers in that part of the industry have seen a parent at an interview. It’s still only the 1 parent, but all of the interviewers in the area have seen them, so they all report that they’ve seen a recent college grad bring a parent.