Bottom fields blurrier than top fields

[Katie Boundary]

I'm currently ripping a TV show. A lot of it was shot on video (60 hz interlaced). Other parts were seemingly shot on film at 30 fps and then telecined. In either case, the bottom fields are slightly blurrier than the top fields. For the parts that were thrown together in the editing room, however, like the title sequence, everything is equally sharp.

What causes this?

[therube]

(Cause? No clue. But maybe a screenshot might help others...)