In course 1 week 2 assignment for reshape this code works but the below code does not work. can anyone tell me why is that?

train_set_x_flatten = train_set_x_orig.reshape(train_set_x_orig.shape[0], -1).T

test_set_x_flatten = test_set_x_orig.reshape(test_set_x_orig.shape[0], -1).T

# why not this—>

train_set_x_flatten = train_set_x_orig.reshape(num_px *num_px* 3,m_train)

test_set_x_flatten = test_set_x_orig.reshape(num_px *num_px* 3,m_test)

train_set_x shape: (209, 64, 64, 3)

train_set_y shape: (1, 209)