Python-сообщество

 Почему в данном примере (в функции "minus_last(m)")  j лежит от (0, l), а не (0, l - 1)?
# -*- coding: utf-8 -*-
import random
def make_matrix(m,n):
	for i in range(0,n):
		a = []
		for j in range(0,n):
			a.append(int(random.random() * 100))
		m.append(a) 
 
def minus_last(m):
	l = len(m)
	for i in range(0,l-1):
		for j in range(0,l):
			m[i][j] = m[i][j] - m[l-1][j]
 
def print_matrix(m):
	for i in m:
		print(i)
	print()
 
arr = []
 
make_matrix(arr,4)	
print_matrix(arr)
 
minus_last(arr)
print_matrix(arr)