Looking at the screenshots that redwolf posted, it looks like the Pz IV is in a better position, in that he is sited in or around trees whilst the SU-122 is cresting a higher hill and is therefore silhoetted against the sky, making for an easier shot for the Pz IV. I don't know whether the AI factors this in, making vehicles realize that they are clearly visible against the skyline and therefore pulls them back. Just an observation.