She received an A because the judges changed up the "criteria".
IIRC from watching Produce 48, they realised that the Japanese trainees did not receive the same training as the Korean ones, hence why the Korean trainees were performing better and receiving higher grades - so they changed it up a bit and made it a bit easier for Japanese trainees to receive an A.
I don't think they ever explicitly stated this but it was implied because of above ^ (in the middle of evaluating all the trainees, the judges themselves acknowledged that the training was different blah blah).
i agree but still don't think she should have gotten an A (regardless of the changed evaluation) but that's history...