the implement Dijkstra maybe have issue

hi,  sorry i have question about in the function calc_index(self, node) in class DijkstraPlanner:
why need to minus min_x and min_y? node.y and node.x have been index format already, but self.min_y and self.min_x are meter unit, it seems invalid using node.y minus self.min_x with different units of measurement ?