It was explained to me as a difference in the way certain arithmetic calculation were made in the actual hardware. The render engine was mental ray. It was real

It was explained to me as a difference in the way certain arithmetic calculation were made in the actual hardware. The render engine was mental ray. It was really only apparent if there were atmospherics, such as volumetric lighting, in the scene. Net renders with the workstations participating would come back with some of the resulting tiles looking noticeably different—the density of the atmospheric effects (ray marching) would appear to have differing densities between the alpha-rendered tiles and the SGI or Intel-rendered tiles.