The word for nshima changes with the region: what's ubwali in Bemba-speaking Kasama is nsima in Chewa areas and takes other names again along the Zambezi.
In Bemba, the staple porridge Zambians elsewhere call nshima is ubwali, and in Northern Province it was traditionally cooked from finger millet flour — denser, darker and earthier than the maize version. It's still made this way in and around Kasama.
Why it's made this way: Finger millet, alongside cassava, was historically the main crop of Bemba chitemene slash-and-burn agriculture in Northern Province, long before maize became Zambia's dominant staple, so ubwali remains the more traditional version of the dish here.