I know weight/diet studies are a dime a dozen and you can always find one that ‘proves’ whatever idea you want to prove, but this might actually explain why that is.
Volunteers had glucose monitors implanted under their skin to see what foods made their glucose levels spike. They also had their gut microbes tested multiple times over the course of the study. The results showed that certain foods (eg grains, starches, sugars) made glucose levels spike in some people, but not others. In other people, foods that are considered healthy and low-glycemic made their glucose levels spike. It appears that having different types of bacteria in our digestive system changes how we process foods entirely, and can be completely different for each person. That means that eating a low carb diet might well be really effective for some people, but might make others gain weight. Same with any other diet that focuses on a set diet of a few particular types of food.