Since I've been spending a lot of time transferring shows to DVD and editing out commercials, I'm going to pull the expert card here.

Currently on ION (no local newscast), when I use my DVDR HDD to edit out commercials, there are plenty of times I have to skip forward (1:00) 4 times, and then skip back (0:15) 3 times, for a total of 3:15. I've seen other channels, both broadcast and cable, with similar varying commercial block lengths. I think many times it has to do with the varying total lengths of each episode.