Another PE question

Hi everyone
My son bought a basic fare economy ticket LHR-SYD. When he was at the airport going out, he was charged over 400 to change it to a flexible ticket + airmiles to upgrade to PE.
Coming home he had the same conversation with the airport staff and they offered exactly the same upgrade ticket + miles but for only 200?
Could someone explain why the charge from London is so much more?
Thanks
My son bought a basic fare economy ticket LHR-SYD. When he was at the airport going out, he was charged over 400 to change it to a flexible ticket + airmiles to upgrade to PE.
Coming home he had the same conversation with the airport staff and they offered exactly the same upgrade ticket + miles but for only 200?
Could someone explain why the charge from London is so much more?
Thanks