Well, it is obvious each study reports different amount of sleep. Pretty much as metabolism, the process of sleep differs. I need around 7 hours of sleep for a normal rest, being 7.5 the best (empirically proven). It's all about the amount of energy you use throughout the day, your body's condition (i.e. illness) and how fast can your body replenish itself (teenagers are under the effect of lots of hormones, increasing the biochemical processes and, in consequence, affecting the sleep). It's your own little world after all...