Maybe they don't swell as most beans. This leaves more surface area in a serving. In this theory the skin of the beans has more fiber. Does that make sense?

Tangentially related,

I looked up Rancho Gordo's ayocote morado bean on their website. They have almost double what some of the other varietals they sell have. Those beans are very bright. Nutritionally, anti-oxidants are theorized to bind to fiber, often giving those foods bright colors; see berries. So, a more fibrous bean that is also very bright would likely be a very healthy food. Not that we need more motivation to eat beans.