Insertion sort on small arrays in Merge Sort
The idea most probably is to chose sublist size $k$ such that merge sort likely is faster for bigger problem instances and insertion sort for smaller ones.
$\lceil s = n/k\rceil$ sub-lists may be suboptimal for merging lists of disparate size later on.
One alternative is splitting into $2^{\lfloor\log_2{n/k}\rceil}$ lists.