13.1 includes buses and trucks. Actual for passenger vehicles is 10.6.

https://www.bitre.gov.au/sites/default/files/is_091.pdf:

“The average rate of fuel consumption across all Australian vehicles in 2016 was 13.1 l/100km. However, this includes buses and trucks which are overwhelmingly used for business purposes. Given that this study is focused on the vehicle costs of Australian households (and not businesses), Figure 1 presents the average rate of fuel consumption for the three types of vehicles which have significant private use, namely passenger vehicles, motorcycles and light commercial vehicles. Motorcycles are relatively fuel efficient using 5.6 litres of fuel per 100 km travelled in 2016, compared to 10.6 l/100km for passenger vehicles and 12.0 l/100 km for light commercial vehicles.”