I think while that may be a factor, there is a trend among schools not to fully extend students in too many subjects - I know I had to struggle for my 4u math + 4u english combo. I was warned of the perils and dangers constantly, and that only increased if I brought up 3u history. I can see their point though since if you extend in a number of subjects the red tape can trip you up e.g if a student wants to do 4u math + 4u english + 3u music or history or art or language. Now that gives 11 units but the student *has* to take on more to fill the 4 subject requirement and since not all schools have SOR1 this means you'd *have* to take 13 units if you wanted your extensions and considering that most of the extension subjects require a fair bit of effort then 13 units is gonna be alot to ask of students. Then add in the fact many people are strongest in a certain type of discipline, it becomes unusual to find people who are versatile to back it up *and* willing to deal with the red tape.
And thats Dr.kami's rant/lecture on extensions