Not sure about the Chine Yuan since boxofficemojo has yet to update that part, but regarding the GBP and AU dolalrs, they were both HEAVILY in favor of Avatar, you can check boxofficemojo if you would like for that info, just go to Avatar/TFA in the foreign section choose which country you would like and select local currency, it will then show it's worth agaist the dollar for said period (you must select a date as well). Avatar had a HUGE advantage with the currency exchange rate compared to TFA, if they were tied, the results would be MUCH closer.
In Brazil for example when Avatar released the Dolar was worth +/- 2 Reais. Avatar is the bigest box office in Brazil history with 60M dolars made +/-. Now with FA the dolar is worth +/- 4 reais, and FA still managed 25M dolars so far, that wouldve been 50M on Avatars day. So for FA to make more money than Avatar it needs to be seen twice as much, the fact it still managed 25M with this crap currency exchange rate is a phenomenal achievement.
Thats just 1 country but pretty much all of Europe and the rest of the Americas also have hugely disfavorable exchange rates in comparission to Avatar. The Euro was almost 1.43 dolas on Avatar release thas a gigantic difference from what it is now (its close to 1.09 dolars now).
I checked the historical value of those currencies vs. the US dollar at the time Avatar released and now. I didn't do it on boxofficemojo so my guess is that boxofficemojo is wrong.
If box office mojo is wrong than Avatar numbers are wrong and it made less than 2.7 billions WW since Mojo has the official numbers for what the movie made on each country in local currency and than just converts it to dolar. Its obviously not wrong.
And I checked historical rates as you said just to be sure, on 2009 during Avatar release 1 Australian dolar was worth 0.84 US dolars, on december of 2015 (FA release) 1 Autralian dolar was worth 0.73 dolars, so adavatge for Avatar. And the British pund was 1.65 dolars each in August 2009 and 1.49 dolars each on December 2015, yet again favorable to Avatar. The Yuan is very very slightly favorable to FA (0.146 for Avatar x 0.153 for FA) that barely even make a difference, FA would gross as much as Avatar with like 95.4% of its tickets sales.
Looking at these exchange rates closer like Im doing now, since your post brought it to my atention FA realy has to sell way more tickets than Avatar to make up for the hughely disfavorable exchange rates WW, maybe even around the 1.3-1.4x more tickets mark. If doing the completly nonsensical ludicrous thing of considering that a movie that sold like 100 million tickets 70 years ago would sell the same amount nowadays and just adjust to inflation to see how much money itdve made is a thing, adjusting exchnage rates to match the ones more favorable to a certain movie should be a thing to lol.