Can you please explain why I need a visa and why I do not qualify for the 72 hr stopover in Beijing ?

Dear Sir/Madam
My wife and I are flying air china from Melbourne to London with a stopover in Beijing, the flight to Beijing and the return flight from Beijing to Melbourne transit very briefly in Shanghai.
My stopover is Beijing not Shanghai .
Air china flights from Melbourne to Beijing all transit first in Shanghai, flights from Sydney are direct to Beijing thus qualify for the 72 hr stopover. To me it does not make any sense at all.
Your thoughts will be appreciated.
Thanks Vange