I think just as we all have different metabolisms, those same metabolism react differently to exercise. Some people are able to exercise and have it make a big difference, others don't seem to get dramatic results, but it does help them maintain their weight.
I think one of the benefits of low carbing is that it lets people get down to a weight where they can exercise, without threatening their joints or heart. Many people here have mentioned that once they start feeling better, they want to exercise once more.