While we tend to assume all shelter dogs and strays must be highly mixed, we do often see rescues return as single-breed dogs! As surprising as it may seem, single-breed dogs are often surrendered to shelters, especially when they don’t have the right look for the breed.
While we can never guarantee a dog is 100% anything, we are very confident in our assignment of single-breed ancestry. The DNA of a dog considered "single- breed” should almost certainly have at least 95% ancestry from that breed, and likely even 100%. Even a 1-5% mixture with another breed (usually undetectable by a DNA test) can yield some surprising looks in an otherwise single-ancestry dog
Keep in mind that breed standards do not account for the significant phenotypic variation we can see within breeds. Many single-breed dogs do range far outside the breed standard- this does not make the dog in question any less of that breed!