python の言語的特徴？

sample.py

b = [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
b[2][3] = 3
print(b)

# => [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 3], [0, 0, 0, 0], [0, 0, 0, 0]]

a_num = 4
b_num = 5
c = [[0]*a_num]*b_num
c[2][3] = 3
print(c)

# => [[0, 0, 0, 3], [0, 0, 0, 3], [0, 0, 0, 3], [0, 0, 0, 3], [0, 0, 0, 3]]

どちらも同じ配列を作っているのに、配列の置き換えがうまくいかないのはなぜ？