“Whereas most American colleagues participate for the full duration of a meeting, other American colleagues come and go. Why?”
There is a very pragmatic answer to that question: Americans will participate in those parts of a meeting which are relevant to their work. They come for that particular hour, and then leave afterwards.
The flip-side of the above question from the American perspective is: Why do German participants stay for the entire duration of a meeting? Aren’t they sitting through discussions that are not relevant to their work? Isn’t that inefficient?