Answer:
Because of the muscle's mass that are used for each grip.
Step-by-step explanation:
EMG or electromygram is a technique that is used to measure the amount of electric activity that occurs in a certain muscle during its contraction etc.
During the overhand grip, the outside of the hands are facing outwards which means that we are using the biceps more heavily but during the underhand grip, the palms are facing outwards and the triceps are used more heavily.
The strength and mass of these two muscles are generally very different and the difference in mass results in the difference of electrical activity in the muscles, causing the EMG results to be different between the underhand and overhand grips.
I hope this answer helps.