I assume you were charged an extra $834 to add those flights? When you make changes to a booking the taxes for all flights, not just the added ones, are recalculated at today's exchange rates, but usually that doesn't make a lot of difference overall IME.
You have a stop after each flight so I would have expected the taxes would be calculated for each flight separately, then added together. So, I would think your figure of about $400 should have been pretty close. Connecting flights can be different - the best example of that being the UK's higher airport departure taxes you have to pay unless you are transiting. Also, the taxes can vary depending on what direction you take a flight in.
I got figures of 41 & 29 Euros for your 2 IB flights individually (total of 70E) but when I did them as a multi-city the total taxes were 95E i.e. about a third more. The CDG-HND flight came in at 190E. Not sure why there would be that difference for the IB flights but I'm not up with all the nuances of how QF does their calculations. It would be good if someone more knowledgeable could post some info.