Solar Diffuse Radiation Model on an Inclined Surface

I have implemented the Perez 4 (as numbered by D., Yang 2016 ) diffuse radiaition transposition for a solar PV model. It seems to work but I have no way of knowing if it really is.

My panels have a vertical tilt, are at a latitude of -79.5°. The Perez 4 model produces peaks of 1.9 and a mean value of 0.91 where an isotropic model would have a constant sky view factor of 0.5.

Is this reasonable or is there something wrong with my implementation? I had not expected the mean of the Perez to be so much higher than the isotropic.