Yes - courses with high scaling had students who tended to perform well in all the courses they took, and vice versa for courses with low scaling.
I believe that, in the past (before 1986), courses were scaled according to how 'difficult' or 'easy' they were. The trouble is, such a process is completely subjective. Some courses are easier for some students whilst being harder for others. Although the current system isn't perfect, it doesn't have any subjective elements, which theoretically means fairness for all.
3u Computing Studies was scaled better than SDD in the past. It had scaled means of 29.5 and 29.2 for 2000 and 1999 respectively, whereas SDD had a scaled mean of 25.9 last year.
The syllabuses are almost identical, although I think 3u Computing Studies had a slightly greater emphasis on algorithms and programming.
I suppose I should also point out the difference in candidature now. There were 3379 students undertaking SDD last year, but 3u Computing Studies had only 1874 and 1782 students in 2000 and 1999.
Under the old HSC, only the crme de la crme were game enough to take on the 3u and 4u additional courses, and so logically those courses would have been scaled higher. Part of the new HSC reforms have been to encourage students to take on courses at the higher levels.