There are many excellent reasons why someone, who has a perfectly valid excuse for it, might NOT want to "tell the whole truth" about a year's gap. There is a bit of nuance here, specifically the acceptance that something which isn't the full truth DOES NOT necessarily constitute a lie or deception.
The reasons might be be personal or perhaps the interviewee doesn't want to be subject to the judgement of a stranger who will smugly decide whether or not the reason was "valid".
Many students get burned out in the middle of their college careers, many want to try something in their life while they're still young. Perhaps "the reason" is embarrassing. Perhaps they had to take time off because of lack of funds.
Unless one is getting a lifestyle screen for a top-secret job, somethings are just too much information in an interview. The risk is that it could be a distraction.
If you're absolutely not comfortable with fully explaining the gap in your college years, just come up with a something vague, albeit explainable. Reasonable people know that students in their teens/twenties are still "on the launchpad" and things don't always go according to plan.