"How strange that AM bronze does not progress into Am silver, it sounds like two different dances?"
In many ways they are two different dances.
But even if you look at what really are two different dances, such as international waltz and foxtrot, you find eventually that there is a lot of technical similarity between them, which helps a lot with the learning process. In the end you have one concept of dancing, with each dance making more use of some of the common techniques than others and inflecting them a little bit with dance-specific character. That's especially true at upper levels where waltz and foxtrot borrow so many elements from each other (when you get passing elements in waltz, and some types of closing elements in foxtrot), though hopefully still execute them with the flavor of the dance being done.