From a person who keeps in with the HK DVD world...
Sometimes, HK companies 'upgrade' their DVDs. This doesn't happen very often, but it has been known to happen. They don't tell people, and just start pressing a better quality version of whatever set they're releasing. What I think happened is they started using better quality sources after volume 3 hit, so they were just in time to make Part 1 of better quality.
Now, as you probably know, Pretty Guardian volume 1 (HK) is Episodes 1-8 (and the special), and the volumes afterwards are consequently 5 episodes each. (9-13, 14-18, 19-23, etc. etc.) Well, where the DVDs are at right now, episode 33 (which just aired) will finish off volume 6. Meaning, shortly after volume 6 is released (HK, remember), Part 2 will be as well.