Dear Earenst,
The beam irradiance shading factor reduces only the beam (direct) component of the solar radiation incident on the array. The diffuse irradiance still reaches the array so that it may generate power even with a beam shading factor of 1.
Also, I can't tell from the graph, but it is possible that the system has more than one subarray. If a system has more than one subarray, and only subarray 1 is shaded, then the other subarrays would contribute to the array's total power output.
Best regards,
Paul.