I actually believe that the DVD's are downrezzed from 1080i masters that were made around 2000 when there still wasn't a set standard resolution for HDTV.
http://bluray.highdefdigest.com/2469/startrek6_undiscoveredcountry.html
The transfer is also riddled with aliasing in fine details, which point to it being sourced from a 1080i master that's been de-interlaced at the studio.
The quality looks more like it came from a 720p master, which was more in use back in the 2000 era as 720p was seen as a bridge between standard definition (Enhanced Definition - EDTV)and (in 2000) the future high definition standards. Plus back then studios were also producing shows in standard definition, but shooting them with the ability to upres the image to 720p for broadcast (just look at what Warner brothers did with Smallville Season 1, originally the show was aired in 4:3, but everything was edited in standard definition 16:9, and even now the standard definition 16:9 masters have been upressed for 720p broadcast and Internet download---but the quality is not fit for Blu-Ray. Also Warner's offers to TV stations HD versions of Babylon 5 that are HD versions of the DVD's, but the HD versions have existed since at least the beginning of Season 5 as TNT aired the show in HD.). So the aliasing issues could be from