I wonder if it's a case of soluble vs insoluble fiber. Insoluble fiber is often described as speeding up transit through the intestines, which might leave microbes less food to eat, or increase the microbes being driven out before they can establish.
I had a laugh years ago when I realized that jam and jellies are full of soluble fiber and my entire childhood my parents were sneaking fiber into my diet. My whole childhood was a lie!
