This definitely doesn't come as a surprise to me. As I've gotten older, I've grown more "in tune" with my body, and I feel fatigued when I'm lacking sleep. Back when I was a young teenager I felt the same every day, regardless of how much sleep I'd gotten. Now I always take my vitamins every morning, make sure to hit the right food groups -- otherwise I feel super crappy.
What about polyphasic sleep? If the study only look at normal sleep, then obviously the ones who sleep less than 7 hours will be less likely to remember things than the ones who had a full night sleep. REM sleep is important, but REM sleep does not cover all 7 hours (more like about 2 hours). So if you get to REM sleep faster, you won't need to sleep as much.