I’ve seen a few tests where MPEG2 is better at maintaining all details across the picture, while AVC/H.264 better maintains detail that the human eye is likely to see. That is why smaller things like shadow detail and potentially background items that are not the main focus may be better preserved. While you’d imagine that these smaller details aren’t as necessary, they add an extra level of depth and are very useful if you need to upscale, since the extra detail in those areas provide better approximations for what the image should look like at a larger size.

http://www.streaminglearningcenter.com/articles/h264-vs-mpeg-2-quality.html shows what I mean - look at the buildings in the background and the people in the crowd. That website does not show them at the same bitrate, but you can still see how the bitrate distribution differs.