Essentially what happens is:
Say your mark in Chemistry was 50/100, whereas your mark in Math Extension 1 was 40/50 (just to keep it relatively simply). As Chemistry is a 2-UNIT subject, the mark out of 100 is halved, to form two marks out of 50 (in this case, 25/50 and 25/50). As Math Extension 1 is already a 1-UNIT subject, it does not need to go through this progress.
Okay, the next step is consider what units will actually count (ultimately, two of the above units WILL count; either ONE of the two units of Chemistry and Maths Ex 1 OR, both of the units of Chemistry). However, considering the candidate's performance in Mathematics Extension 1 outshines that of Chemistry, only ONE of the two units of Chemistry (i.e. one of the 25/50's) will count. The second unit of Chemistry will be 'remedied' by Mathematics Extension 1 mark out of 50, to form a new mark out of 100 - 65/100 (this was calculated using one of the units of Chemistry (25/50) and the Maths Extension 1 mark (40/50) to form a total out of 100).
Comparatively, this is much better than a 50/100 in Chemistry. I believe the Board of Studies then subject the half of the marks (i.e. 25/50 and 40/50) to scaling/aligning and a bunch of other complicated and somewhat unknown processes to form your HSC mark. Thus, 11 units is RELATIVELY safe.
At the end of the day it doesn't really matter. I do 11-units; what will happen will happen. However, it must be noted that your BEST 2 units of English will always count (so if you do Extension English, it may 'remedy' Advanced English, or vice versa, in the same way as aforementioned). Hopefully this made sense, it's kinda late.