Well, that could be because a lot of people waited 1, 2, or 3 years out of college before applying because their college applications were so weak. It's not really clear from this data if, at the point at which applicants can be considered "nontrad", there is actually a boost to being nontrad. It's worth keeping in mind that true non-trads make up a small portion of the pool so they wouldn't have as large of an impact on the data as the swaths of students aged 22-25.
Personally, as someone taking 2 gap years, I think it's been much easier for myself and others with similar gaps. It's much easier to articulate my views, accomplishments, and goals while still being taken seriously. I haven't been asked questions like "why don't you do X?" or "don't you think you'd be better suited for Y?" because I've taken some time to explore other options and can speak confidently about what I do and do not want out of my career. It's a nice gig and I'm not even "nontrad" in my opinion.
Even still, so many premeds will continue to refuse to take any time off because of some perceived "race" that they are a part of with the rest of the medical establishment. Some have pretty good reasons for not taking any time off. Many have probably never thought of it. Others simply don't know what to do without staying in the confines of the comfortable, defined "track" that becoming a physician entails. In the end, it doesn't really matter that much how you get there so long as you handle your business, nail your academics, and try to pursue things that are meaningful to you.