Why are op.rotation_matrix in PointGroup.symmetry_ops not orthogonal matrices?

[blondegeek]

I'm using the PointGroup class to get the vector representation of the group $D_{6h}$ (6/mmm) and noticed one of the generators is not an orthogonal matrix.

D6h_group = PointGroup(point_group_dict['D6h'])
np.array(D6h_group.generators)[-2]
>>> array([[ 0, -1,  0],
       [ 1, -1,  0],
       [ 0,  0,  1]])
np.array(D6h_group.generators)[-2] @ np.array(D6h_group.generators)[-2].T
>>> np.array([[1, 1, 0],
       [1, 2, 0],
       [0, 0, 1]])

Interestingly, if you do use these generators, you do construct a set of 24 matrices that do yield the correct multiplication table for the group.

What's going on? Can someone provide a reference for why these matrices are being used instead of elements of O(3)? Thanks!