I think my body acidentally got advanced as I started to get an hour earlier than usual these days, resulting in sleeping 7 hours not 8. Why?

It's all good. Adolescent sleep cycles are still immature. Most adolescents need more sleep than adults and need to sleep in later. That is the basis for the AAP recommendation for later school starts. Your change in sleep need appears o b the result of your personal sleep cycle attaining the next level of maturation.