I'm not 100% sure, but I don't agree this happens all the time. AM will apply lesser of rule:

For instance:

HKG-SYD in PE, SYD-HKG in J:
As one ways: 30,000 + 45,000
As return in J: 80,000
Charged: 75,000

HKG-KIX in PE, KIX-HKG in Y:
As one ways: 24,000 + 20,000
As return in PE: 36,000
Charged: 36,000


This only applies if the classes were split initial booking though.
Another example, I have confirmed HKG-SYD PE - 54,000 miles
J has opened up on the outbound - I will be charged 80,000 to make the change even if only one-way has changed.