From a thread about these Blu-rays:
They filmed the whole series using cameras way ahead of their time. The airing versions of the show were cropped to 4:3 and reduced for televisions at the time. But since due to how they recorded it, there were much higher quality versions of each of the episode they had stored away they thankfully kept on hand, and good they were insightful enough to think the quality would eventually matter in the future to record like that.
The much higher quality versions started popping up in some weird countries like India in 2011-2012, but only in very odd regions with X-Files re-runs. The whole series was updated on US Netflix earlier this year to include all the HD versions of the episodes in preparation for the new season coming out next month (weird to realize it's a month and a half away). I know Canada recently got the HD versions on Netflix too. And this is the first time they've been sold (and due to being Blu-Ray, probably look a bit better). I've been re-watching the series on and off the last several months with a friend, we just finished Season 4, and I can say those look great and watching the whole series with the HD versions is worth it, and I imagine the Blu-Ray looks nicer. But it seriously looks good, it looks like the series could of been filmed within the last few years, not 22 years ago.