[thaumasiotes]

Just based on eyeballing the graph, I'd say there's a cultural element to what codes get used, because individual clinics often show more or less activity at a particular code for all six years. Choosing a code is something of a gray area, so that's not necessarily malicious, but I think "whoever walked through their door got treated for whatever random thing they had" is slightly oversimplified -- the patients will have been treated appropriately, but local culture will have pulled them into being coded in certain ways over other, arguably equally-applicable ways.

(Clinics having their own "personality" in coding could also be explained by the clinics having locally well-recognized specialties. That's hard to evaluate without knowing which codes are which.)