Correlation rather than causation, would be my first thought. A randoml vegan is vastly more likely to be getting regular exercise and be generally health-conscious than your randomly selected traditional diet person. This study tries to downplay non-dietary lifestyle factors, but can't help conceding that:

> Vegans tended to report higher levels of physical activity; there were no clear patterns for education level across the diet groups; vegans were least likely and meat-eaters most likely to be married and almost the whole cohort reported ethnicity as white. Nulliparity was most common in the vegan women and least common in meat-eaters, with meat-eaters tending to have a greater number of children than the other diet groups. Some of these differences in lifestyle factors between the diet groups are likely to result from the disparity in median ages.

So, more exercise, less marriage, fewer children, and younger age for the vegans...