I'm unsure why that would be unexpected. I suppose the word vegan brings forth the idea of a large variety of fruits, grains, and vegetables however vegan itself doesn't mean eating a variety of plants, it means not consuming animal products in any form.

Additionally, since bacteria strains tend to be specific to their environment, i.e. the plant that they're living on (which the plant itself thrives better in particular environments), it makes sense that a diverse diet would lead to a more diverse gut biom.