Math behind scipy.ndimage.convolve

While I have already found the documentation on scipy.ndimage.convolve function and I "practically know what it does", when I try to calculate the resulting arrays I can't follow the mathematical formula. Let's take for example:

a = np.array([[1, 2, 0, 0],`
             [5, 3, 0, 4],
             [0, 0, 0, 7],
             [9, 3, 0, 0]])

k = np.array([[1,1,1],[1,1,0],[1,0,0]])

from scipy import ndimage

ndimage.convolve(a, k, mode='constant', cval=0.0)

# Why is the result like this ? 

array([[11, 10,  7,  4], 
       [10,  3, 11, 11],
       [15, 12, 14,  7],
       [12,  3,  7,  0]]) 

I would appreciate a step by step calculation.

Thanks in advance!