at rest, so doing nothing but breathing. That's a 1000 calories deficit. 3500 calories deficit per week would lead to roughly a pound of fat lost per week. So 1000 a day would be two pounds. Almost no one is at rest all day, and if the starting weight is more, the body burns even more at rest, so bigger deficit = more weight loss per week.

Maybe they want people to see results early to keep them motivated. Not a bad idea. I don't know though if I could make it through the day on 1000 calories.