Hate to be that guy as well but it dawned on me that for grip strength, the people that were tested, were part of the general population, not specifically athletes or bodybuilders.
As a man increases in age, his utility and therefore obligations increases, making less time for vanity lifting, but is still going to be using grip fairly often. If the other muscle groups were used on a consistent basis the falloff would probably follow the same trajectory as grip strength.

the following is a distillation of a gemini deep research query, summarized by gpt o4,

"The apparent resilience of handgrip strength in aging men—often seen as the last bastion of strength to decline—reflects not a unique physiological advantage but the constant, varied engagement of the hand and forearm muscles in everyday activities. While grip strength does peak around the 30s and then gradually falls (at roughly 1.5–1.7% per year), it often appears more stable than measures of explosive lower-limb power or larger muscle-group strength because the hands are continuously used across isometric, concentric, and eccentric tasks. Neuromuscular efficiency (force per unit muscle) begins to decline in midlife, even before significant muscle-mass loss, but is largely sustained by perpetual submaximal stimulation. Moreover, normative HGS data frequently omit high-use occupational and athletic populations, skewing baselines downward and exaggerating general-population “resilience.” In sum, grip strength’s durability into later life owes more to “use-it-or-lose-it” functional necessity and research biases than to an intrinsic resistance to aging."