In playing around with another RTW today, I noticed a *massive* spike in starting CAI-AMM-DOH-BOS as opposed to CAI-LHR-BOS. Price jumped by $3000 AUD.

Very odd. I'm building the following routing:

CAI-AMM
AMM-LHR-BOS (or AMM-DOH-BOS)
BOS-LAX-LAS
LAS-PHL-BOS
BOS-JFK-AKL
AKL-DOH

If there's only one QR longhaul flight (DOH-BOS or AKL-DOH), the final price is $6545 AUD ($4467 AUD base and $2078 AUD taxes). That's with AMM-LHR-BOS.
If I switch so there's two QR longhaul flights (AMM-DOH-BOS and AKL-DOH), the final price jumps to $10268 AUD ($7906 AUD base and $2362 AUD taxes)

Adding a DOH-CAI flight on the end reduces the fare by about $700 AUD.