Disclaimer ..... I haven't been involved in any mars testing. LOL. The information I know....I've read.
Yes, the test is only 84% accurate and not accurate when testing purebreed dogs. According to the website in order to test purebreed dogs...they would have to test EVERY line out there in a particular breed.
They have genetic markers from 130 akc recognized breeds. It's those markers they are looking for when examing a particular dog.
Here's a FAQ page from the website...
Frequently Asked Questions about Mars Veterinary
A dvd is supposed to become available explaining the genetics and the biewers involved in the testing. I haven't heard if that's happened yet or not.