If the rest of your schedule keeps the same numeric time, then this does make sense. If your schedule allows for 9 hours of sleep before waking up for school, then you're still getting 9 hours. If you changed the numeric value, then it would either 8 or 10 hours since the school start time retains its numeric value.
"I believe most of these studies are performed in the USA."
The study being discussed is from Singapore.