according to chronology, almost a year passed from chapter 8 to chapter 9, and then one more month from chapter 9 to chapter 10 who knows, maybe another year will pass from chapter 14 to chapter 15, and amiya will be even more grown
Each alternate Amiya forms kinda sounds like the character model she copied. Guardmiya make her sounds a bit like Chen, while Medicmiya make her sounds like Theresa
To be fair, her parents died when she was very young, so she spent most of her time with Theresa and Kal who most likely did not have an Australian accent.
Amiya lives in the same realm as Blacknight of 'this character got their EN voice *very* early on, so they have a generically American accent instead of anything regional or distinctive', although it's... not as egregious for Amiya. (Blacknight, if you'll recall, is *so* Aussie that non-Aussies can't understand her when she speaks. This is the sacrifice we had to make for all the later EN character voices having strong accents, I guess.)