I believe dun is like grey, if the horse carries the gene, they express the colour. But people can misidentify, and colours dont always look like the textbook. Dun and cream can be hard to see under grey, smokey blacks and sooty buckskins can look like brown, I suspect some duns might not be very "loud" and could be mistaken for chestnut, or even bay. A lot of horses are registered when they are very young, and we all know how much some of those colours can change from foalhood to adulthood.