As far as I know 9 semesters were usual for Diplom in nearly all subjects (note that some degree courses such as medicine, dentistry, pharmacy, law that are more thightly controlled by the government to my knowledge still finish with a Staatsexamen (state examination) degree). The 10 semesters for Diplom in computer science (1 more than usual; 9 are usual) are to my knowledge somewhat unusual in Germany and the reason is that at my university that it was intended (as a pilot project) that you write your bachelor thesis while you do a 6 month internship at a company that does the "daily tasks" of the supervision of the bachelor thesis. For this one additional semester was introduced.
