I don't know if this is true or not at other shelters, but I know at the one I volunteered at for over a year, if they couldn't tell what the dog was mixed with, or if they were told and didn't want to put that on their card, they would make something up.

Now, I have a very good eye usually for spotting what some mix breeds are. If I knew, or I looked at them and thought they had this or that in them, I would mark out or white out what they put and put what I thought.
One time a beautiful pit bull/siberian husky mix came in and the owners told us exactly what she was mixed with.. They put on her card "Bulldog/GSD" on there even after the owner wrote down on the turn in papers what she was. I went and changed it..

I worked in the main part of the office where they took in the dogs, and did adoptions and etc... Plus I helped clean the dog and puppy cages.

Whats really sad though, with all the lab/pit mixes in shelters, there is someone in my moms area that is actually breeding their labs with their pit bulls on purpose and calling them lap bulls.