Yep, there's variation. You lose the most the first 2 weeks (see the sticky on ketosis).
I'm happy to lose more than a pound a week at this point, but I think Medifast's claim of 2-5 pounds a week is only true for 300+ pound men. I NEVER lost 5 pounds in a week, even my first week, and I only lost more than 4 pounds in a week TWICE in 15 1/2 months.
I've also noticed that sodium affects me a lot on this diet. I eat so much less sodium on a daily basis, that 1 meal at a chain restaurant can put me up 2 pounds or more overnight.
Hang in there - the great thing about this diet is that you almost always lose.
Lost the first 100 with Medifast, now working on the last 25