They've been doing the numbers the same way for years but essentially while there are 103 scheduled auctions the clearance rate is calculated solely on the results reported. So out of that 103 maybe only 70 report the result be it sold, withdrawn etc. The pessimists will always argue this is manipulating the stats and the 30 odd that didn't get reported didn't sell and it's all a huge conspiracy but despite all this the market is still climbing and has done for years while they beat the same drum. In your table those that are re-scheduled/moved to private treaty aren't part of the stats for the week anymore.
The vast majority of property sales aren't at auction anyway. Last week for NSW there were 743 auctions scheduled but 1985 private sales occurred. Looking at private sales and how long the average ad is listed for gives you a greater insight into the broader market than clearance rates. Even in Sydney there are plenty of suburbs which holding an auction is abnormal ie might make up 1 of 10 properties for sale. The inner city ring and certain areas based on demographics favour auctions but get to the outer metropolitan areas and auctions are very uncommon.
