They aren't losing 8-15 pounds in one week. Trust me.
Many times the shows are not taped every 7 days. Also, high weight losses are much more likely in someone who is EXTREMELY oveweight as well, for the first few weeks. It isn't uncommon for someone 300 pounds to lose 7 pounds their first week, but much of this is water weight. It won't happen though, once they get 20-30 pounds down the road. It also won't happen for someone who is about 155 pounds, trying to get to 130. The smaller you are, the slower it will be, in most cases.