From the MediaPost :
"One surprising result from the data showed that commercial ratings at the end of some shows--"West Wing," "24," "7th Heaven," and "NCIS" --actually get better ratings than the programs themselves. How could that be? Sternberg says it's because viewers are getting an early start to a higher-rated show that is leading out of those shows. For example, "Law & Order" starts at 10:00 p.m., and has been leading out of "West Wing" on Wednesdays. "L&O" also typically does a higher rating than "West Wing." Those "L&O" viewers begin watching a few minutes early, and that includes commercials in "West Wing.""