It may be that your cable company is being preferential to their own cable boxes. SDV is supposed to drop a channel if no one is using it and the bandwidth is needed by something else. Before doing that, it's supposed to send a message to the box to have it pop up a dialog "Are you still watching? Yes or No"; if you don't answer within a certain time limit it can decrease the count of people watching by one. It's not so strange that TiVo is letting the channel go, but that your standard cable boxes aren't.