I have two numpy arrays of the same shape. I want to subtract once from the other. I have tried two methods but numpy is not returning correct values.

dif = np.subtract(b4, b5)

and

dif = b4 - b5

However, for both,

b4.shape == b5.shape
Out[66]: True

b4[0,1]
Out[67]: 72

b5[0,1]
Out[68]: 111

dif = b4 - b5

dif[0,1]
Out[70]: 217

dif = np.subtract(b4, b5)

dif[0,1]
Out[72]: 217

The real answer is 72 – 111 = -39. Why is numpy returning 217?