I think that was not the case before 1.07 patch. They were random. Do you have that DLC of Dynasty Shields II? I thought thats why it is like that in my games, but might be just the patch. All historical dynasties seems to have always the same CoA, Some in Balkan area like Dukljan or Trpimirovic never actualy had there own dynasty shield since in the time when they rulled herarldy was not developed yet, so look like they c hose to have standard but made up for them too (allthough I am not sure how accurate they were even with those who did have a real Coa in there time).
That all said, I do not think you can change even within ruler designer, but if you open save game you will se somevere under the dynasty Trpimirovic numbers as a code for there CoA, you have to master how the code works and type in certan number as how would you like to look like. It could be done by modding, probably very simular like this. This is all I can think off.