When a and b are 1-dimensional sequences, numpy.cov(a,b)[0][1] is equivalent to your cov(a,b).
The 2×2 array returned by np.cov(a,b) has elements equal to
cov(a,a) cov(a,b)
cov(a,b) cov(b,b)
(where, again, cov is the function you defined above.)