You will lose more the first 2 weeks - see the sticky at the top of the page. It's mostly water that your body is releasing as you burn up your stored glycogen. It is certainly possible to lose 13 pounds the first week, but I think that is unusual. I lost 11 pounds total the 1st 2 weeks. I would not expect nearly as much lost this coming week.
Yes, Medifast says you can lose 2-5 pounds a week - I can only hit 4 pounds if I am exactly on plan, with NO optional snacks or condiments. I'm not very happy living that way, so I usually lose 1.5-2 pounds a week.