
Well there are some things that are not open to interpretation liek height limits, colour ear or tail set etc. it will only be the details that can be open to interpretaion and also when judging you are having to put dogs in order of merit. Bearing in mind that all teh dogs will ahve faults to a varying degree it is your interpretaion of which faults and virtues place a dog above or below another in quality.
A breed can and does change if a dog or kennel become very influnetial in everyone going for that type. But genrally there is enough variance within acceptable that will keep a breed on an even keel.
Just as an example say a breed shoudl be strudy but athletic. soem people will elan toawrd the sturdy and youn end up with chunky dogs tedning to the heavy if your not carefuyl. Others will be concentrating in athletic and may end up with some dogs that are a bit light in mass.
As long as both types are still around you can stop the breed from going too far one way or the other.