some comments
Students who have applied to do HSC Extension 2 Mathematics must do the Mathematics Extension 1 exam and the Mathematics Extension 2 exam. There is no 2 unit examination. Both Extension Mathematics examinations are STILL BASED ON THE 2 UNIT COURSE. The only difference is that they should be much harder.
Note a quote on the information package; "The course content includes the entire Mathematics (2 unit) course, the entire Mathematics Extension 1 course and addition..."
So it is still necessary to study the 2 unit course because any Extension course covers the 2 unit syllabus as well, regardless of whether it is examined or not. Also it is often a good idea for schools to have assessments on the 2 unit course for Extension 2 students so if they choose to drop to Extension 1, they would not have to catch up or miss anything.
As far as I know, the Extension 1 Mathematics assessments accounts for 2 UNITS of your total units, while Extension 2 Mathematics test also accounts for 2 UNITS of your total units. This is because each has a mark out of 100.
So for the UAI if you had the lowest scaled mark for one of them (Say Extension 2), that one would not count to the 10 units unless of course you are only taking 10 units. But I doubt that Extension 1 or 2 Mathematics would ever be excluded from the ten units since its course loading and scaling is rather high.
There IS NO ASSUMPTION of an automatic band 6 in the 2 unit Mathematics course since the test is not completed.
Probably the most likely reason that Extension 2 Mathematics students complete the Extension 1 Mathematics examination instead of the 2 unit examination is because of the build up of two courses on top of the 2 unit couse.
For example, Extension 1 builds on to 2 unit course, while Extension 2 builds on to the Extension 1 course. In terms of Extension Mathematics examinations the highest two levels of Mathematics is examined.
So for Extension 1 Mathematics students, they do the two highest levels which are Extension 1 and 2 unit. For Extension 2 Mathematics students, they do the Extension 1 and Extension 2 examinations because they are the highest 2 levels of Mathematics for that student.