Why is my battery life shorter than the box promised?

Because the box quotes a best case you are not reproducing. Manufacturer battery figures are typically measured at a low TDP, dim brightness, and a light workload; the moment you raise the power limit, crank the screen, or run a demanding game, draw climbs and runtime falls — often dramatically. It is the same reason we publish battery as a band at a stated TDP rather than a single number. To get closer to the advertised figure, lower the TDP and brightness; to plan honestly, read the reviewer band at the settings you actually use.