1. A product that has been changed to not use any animal products.
2. A product that already fits within those constraints.
Products in the former group have a history of being noticeably worse than their non-vegan counterpart, so "vegan" carries the stigma of "tastes worse than it looks". That stigma then wafts over to products in the second category.
Other categories of products are more careful to use distinct terms. That's why you see things labelled "naturally fat-free" instead of just "fat-free", for example.