I believe that's because above 5 elements, fixed sorting networks are no longer used. Introsort takes over and dispatches to insertion sort, quicksort, or heap

I believe that's because above 5 elements, fixed sorting networks are no longer used. Introsort takes over and dispatches to insertion sort, quicksort, or heap sort as appropriate. Divide and conquer strategies are used for larger sorts, and the smaller arrays could include the fixed lengths 3, 4, 5.