That's the best explanation I've read, and makes perfect sense. Thanks NattylouOne of the main issues with the DNA testing is that so many breeds of dogs share origins and backgrounds. All the recognised breeds of dog are all one species - Canis Familiaris. This is the most diverse species in the world, based on physical attributes, but still one species. Not like a horse and a donkey - which are related but genetically separate species.
So a DNA test for dog breed looks for "markers" known to particular breeds. So look at some of Bunny-girls' results.
The Bullmastiff, being an English originating, gamekeeper's dog - came from largely breeding together Mastiff and Old English Bulldog. Old English Bulldog even had Greyhound in it's foundation... Mastiff contains ancient European dogs (so potentially some of the large guarding breeds from Europe like Pyrenean Mountain Dogs) and more recently, Scottish Deerhound...
Basenji is a primitive spitz dog type which can share markers with other primitives, so for example - the Dingo. Dingo is also behind the Australian Cattle Dog, the Koolie and the Kelpie... Basenji may also share markers with other Spitz type breeds.
So in theory you could find markers from a vast variety of breeds, even within a purebred.
People have sent in tests from registered purebred dogs and got mixed breed results. The nature of the tests is just not hugely reliable.