Hormonal

Resistance training alters the time course of fed-state muscle protein synthesis (MPS) by increasing the initial peak amplitude but shortening the duration of elevation, resulting in a lower total synthetic response over 28 hours compared to untrained muscle.

As you get stronger, your muscles become more efficient at handling stress, causing the growth signal (protein synthesis) to peak higher but fade faster. To maximize muscle growth, you must consume protein immediately after training to catch this sharp, short-lived peak, rather than relying on a prolonged, low-level signal.