Isn't the airline industry amazing? There are 2 answers to your question:

1. For all tickets, some government taxes depend upon the SITI/SOTO status. For example, GST in Australia applies to tickets sold within Australia for domestic Australian flights, but does not apply to the same ticket sold outside of Australia. Thus a QF flight SYD-PER will be cheaper when ticketed by AA (or any other airline outside of Australia) than when ticketed by QF (the GST amount of 15% is quite significant). This is a completely legal tax loophole.

2. For RTW tickets, each segment can be on any of the 8 (soon to be 10) Oneworld airlines. Rather amazingly each have a different fuel surcharge amount (in fact several different fuel surcharge amounts, as it is often different for trans-Atlantic vs. trans-Pacific, etc.). The OW airlines decided that this make the OWE tax calculation too complicated, and agreed that the ticketing airline will use its own fuel surcharge amount for each sector on the ticket. When this was decided, years ago, the fuel surcharge was either zero or a very small amount, so it didn't seem to be important. Over the years some airlines have decided to increase fares, while others have increased fuel surcharges. For a while the AA fuel surcharge was much less; a few months ago this changed, now AA increased their surcharge, however it is still less than BA, QF, etc. So the short answer is that the airlines are doing this to avoid having to have a computer system calculate the correct fuel surcharge amount for each segment. The "high tax" airline customers overpay, and the "low tax" customers underpay; overall it balances out, but creates an arbitrage possibility for how you buy a ticket.