Jealousy
I would also add that not only do people have different metabolisms, but people respond to the surgery in different ways. I started as SMO, but my surgeon refers to me as a "super responder". He says the only way to accurately predict how effective the surgery will be is if your siblings have had it. If they are super responders, it's very likely you will be. Aside from that, there is no way to predict who will respond in what fashion.
One other thing to add is that even SMO can be a huge range. I was SMO, but just barely, I had a BMI of 50. That's a far cry from someone who maybe 60+ BMI. I didn't have over 200 lbs to lose even though I was SMO. I ended up losing about 185.
Anyway, just wanted to add a couple of things to what was already a lot of great info in this thread.