You can request CIC to provide you all their documentation related to Citizenship programme, specifically all manuals used by CPC-Sydney. It is around 950 pages, 500 after all exempted information is severed.
I do not have any explanations as I am not from CIC, but the main reasons could be that your statistics is incomplete, or that some offices have fewer applications, or different staffing situation. My application was AOR - in process the same day, for example, and it is Montreal. Of course count in all the errors those CIC peeps make, and they make them all the time