Video Collector said:
When rewatching some select episodes (on Blu) recently, I was disappointed to see noticeable DNR in every episode. … I remember seeing a screenshot comparison between the season 1 Blu-ray and HD-DVD, which revealed slightly more DNR in the Blu-ray, but I can’t find it now.
A quick search and I couldn’t find such a comparison (definitely post a link if you come across it again). Could the different encoding, from HD-DVD's AVC MPEG-4 to Blu-ray's VC-1, have been a factor? I once noticed a hi-def screenshot (early HD-DVD days) that looked startlingly grainy and paint-program experimented with various procedures to tone it down. Maybe they did the same.