The limited choice of options does impact the results for sure. It sways you one way and stops you for making other guesses. There does seem to be a certain type of dog breeds and some of the choices are not really "common" breeds.
it isn't a perfect study but it's a start. I have always believed breed ID based on looks is pretty fallible. ANd most people want to guess two breeds as though it's just that when most mixes have at least three or more. And what breed is in there may be regional, some areas actaully DO have lots of catahoula's intact and running around which could lead to more catahoula mixes. In other areas a catahoula mix would be pretty unlikly.
I find it odd that dalmation was on the list of options, sure are not a ton of dals kicking around my area.
Though, we once had a litter in rescue we think was a swissie mix (swiss mountain dog) mostly as we knew there was a stray intact male kicking around the area where the female was found.