Mike,
There are numerous reasons dogs get put up. Judges are just like the rest of us and have their preferences of what they like. So, assuming all other things equal, they will pick what they prefer.
Here are some of the things you can face with judges. There are some judges that are only licensed for one breed. Some are licensed for several breeds. Some are licensed for all the breeds in one group. Some all licensed for multiple groups. Some all licensed for all breeds.
Some judges are breeders too, maybe in your breed/group, or not. Some have field experience, some don't. There are other variables I haven't mentioned.
You then run into the judges confidence/experience of what they are judging. In a breed with large entries, like labs, and judge who may not know the breed that well will look at the large number of entries and assume that the majority must be correct. So, the one dog that is different, and possible the most correct, is over-looked. Or, you have a breeder judge that prefers the type they breed and put up what looks like that.
Then you have the whole issue of professional handlers. A judge, who may not be greatly experienced with a breed, will see a big-gun handler on a certain type of dog and assume that is what the breed should look like. So, they use those types.
Then there is the aspect of dogs being heavily promoted/advertised in trade publications. The judges see that a certain dog has so many BOB, Groups or BIS placements and assume it must be what the breed should be like.
So, it is easy to see how the type of dogs being shown can move towards what is being put up which pushes the "off-type" even further. It is kind of like when a certain field dog produces multiple FC's so they get used as a stud widely. People look for FC's and not breed type and you end up with dogs that win FT's but look like a greyhound, for example.
Once the breeding programs go to only produce winners, regardless of the venue, it is easy for dogs to drift away from the breed standard. The problem is the breed standard was created to describe what the breed is. Once most dogs shift from the original it is difficult to move back towards it. It takes a concerted effort on most the breeders part.
One tool show judges have at their disposal is to excuse dogs that do not have merit. When we were showing I never remember this happening.
Tom