I always choose to pay in AUD on online platforms where I can. I presume other people also prefer their own country's currency when it's available, amongst other things it lets you avoid your bank's exchange fees.
It seems like this is a mistake on JLCPCB: all it does it change the invoice to be in AUD (based off a guessed exchange rate), the charge to your credit card will still be in USD (with your bank's exchange rate then converting it to AUD).
I find this rather weird and unexpected. Maybe it's commonplace? I was able to get a corrected USD invoice by manually asking JLC's support, but they also said this:
1, Currently we can only support USD, the invoice shows in AUD because you choose that currency.
So in the future if you want to know the amount in USD, please kindly set it as shown below~
So in other words: you should only ever choose USD on jlcpcb.com, everything else just makes your invoice less accurate (or in my case for reimbursements: flat out wrong).