Comment by wharvle

In a society where these diets buck the norm, I expect some of this effect is just from paying more attention to what one eats. It's a major mechanism behind lots of restrictive diets working for weight loss—paying more attention to food helps all on its own.

I bet there are people out there who eat semi-vegetarian diets basically by accident, who wouldn't think to label themselves semi-vegetarian, and that the beneficial effect is less pronounced among them.