Do all ATVs measure their usage in hours or do some do miles?

I'm looking to purchase a used ATV and the one I was looking at was a 2001 Polaris Sportsman 400...this would be my first ATV and while I was researching I saw that people were saying ATV life is measured in hours...when I asked the lady how many hours were on it she said 3647 miles...I was wondering if this could be accurate and this actually has 3600 miles on it or if she was probably mistaken and it's 3600 hours.