I think some shows are intentionally not including show data specifically to get DVR's to Record it Just-In-Case.
It's kinda rude.
Alas, that is not true, because the guide data I have from Gracenote (Tribune) was perfect.
I think Rovi/TiVo simply doesn't have enough manpower in their guide division - generic data is often farther ahead showing they never had time to properly enter in episode information. Basically they enter the show information first, then fill in the episode information later on. This is because most channels have fairly predictable show schedules so you can create a generic grid of shows then fill in the details later on.
Heck, I sometimes find the grid filled in with shows that aren't airing (show is between seasons or off air) because they weren't updated. Of course, the grid was just generic with no episode information. This happens when there are massive upheavals in the TV schedule, usually around fall when you switch from summer to fall schedules, and in the spring when seasons ends and the summer programming starts.