You have a good eye...and a good monitor.
It could be the set-top box but I suspect it's the newer compression scheme: MPEG-2 vs. MPEG-4. One scheme is better for quality the other for efficiency.
MPEG-2 has less compression and impeccable image-quality...but uses more bandwidth.
MPEG-4 has more compression and less bandwidth...but inferior image-quality.
If you've fiddled extensively with your video settings, I suspect you've already found the 1080p resolution: Settings > Device Settings > Video Display > Video Output Resolution. That's all I can recommend.