The first array has shape (3,4) and the second has shape (3,). The second array is missing a second dimension. np.array is unable to use this input to construct a matrix (or array of similarly-lengthed arrays). It is only able to make an array of lists.
>>> a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])
>>> print(a)
[[ 1 2 3 4]
[ 5 6 7 8]
[ 9 10 11 12]]
>>> print(type(a))
<class 'numpy.ndarray'>
>>> b = np.array([[1,2,3,4], [5,6,7,8], [9,10,11]])
>>> print(b)
[list([1, 2, 3, 4]) list([5, 6, 7, 8]) list([9, 10, 11])]
>>> print(type(b))
<class 'numpy.ndarray'>
So they are both Numpy arrays, but only the first can be treated as a matrix with two dimensions.