I noticed this as well, and I believe someone else posted something very similar to yours asking the same question before. I think it is because this surgery, by itself, is still to new. There is no standard because there hasn't been enough time to form one. The surgeon's are still learning the in's and out's of the details. I'm sure in 10 years there will be more consistency, like you would find with other general surgery procedures.
I think the people who are most successful are the people who took the time to do their research and chose the method that worked for them. I have learned that I am very carb-sensitive. If I am over a certain amount I don't lose, but the same is true for being too low. I know my limits and stick to them. I also think the people who are the most successful are the ones who make active changes in their beyond what the sleeve does. The sleeve doesn't prevent us from drinking milkshakes, etc. The people who are successful avoid these things for the most part. They simply learn how to do the treats as that, as treats. At least that is what I am trying to do.
Don't forget if you are around the 3 week mark, you are most likely going to stall soon. That seems to be normal no matter what diet you are on. I think it is the body catching up to the surgery, like your body just said "oh shoot, what the heck??" and kinda slows down until it gets used to it, then you start losing again. You could also be a stair-case loser... where you stall for a couple weeks and then drop and then stall and then drop. Don't be too worried about the scale, just remember it is a marathon and not a sprint!