Here's why:
* Dun and buckskin are both base coat colors. They are not separate colors, but rather descriptions of how a base coat (like bay or chestnut) is modified.
* Dun is characterized by a dorsal stripe, zebra stripes on the legs, and often a "primitive" look with a darker mane and tail.
* Buckskin is characterized by a golden coat, often with a black mane and tail.
Therefore, a horse can be either dun or buckskin, but not both.
It's possible for a horse to have a dun factor AND be buckskin. This means the horse is a buckskin with dun markings (like a dorsal stripe). However, this is not the same as being "both dun and buckskin".