I guess it has something to do with the fact that the horizon is physically a very long distance away compare to the HUD and a slight displacement doesn't change much. But surely the HUD itself phyiscally rises and sinks as the plane banks? Does this not give some error?
As the horizon sinks in the HUD (like in the 2nd picture), does the artificial horizon sink AND also the "bird" that shows your pitch attitude? Or does it appear still very much like the 1st picture in which real and artificial horizons seem to rotate around the central point of the HUD (providing you're at 0 degrees pitch)?