Just to illustrate how much difference YQ can make, today AA priced for me DONE4 OSL-xDOH-BNE-NRT-YVR-SEA-LHR-OSL.
A very helpful AA person spotted from experience that the Australia-Japan segment had high charges. By changing from QF BNE-NRT to JL SYD-HND, the fare dropped by $921 per person!
Interestingly I also priced a BA-ticketed OSL-LHR-HND-SYD-YVR-SEA-LHR-OSL to see whether the above QR-ticketed itinerary was attracting unusually high YQ. There was only about $100 difference.
These YQ costs are a real mystery to me.