Maybe USA people DO cough differently ?!, page-21

I hope this goes someway to steer your conclusion in the right direction

A cough does not carry an accent. However, the vocal tract would amplify and dampen certain frequencies of noise during a cough that are in line with what you would hear in speech just as if that person were phonating. This is due to the shape of the resonating chamber that is the vocal tract that is individual to each person. So if you know the person, you might be identifying that person based on the resonant characteristics of that person's vocal tract that you may be familiar with through speech.

This is the reason for different results from the company..

The SMARTCOUGH-C-2 study also highlighted the challenges of acute respiratory disease diagnosis, with our clinical adjudication process showing that in only two-thirds of cases did the first two clinical adjudicators agree on a disease diagnosis. Our results for pneumonia and bronchiolitis were lower than expected, however we have now identified clear differences in the clinical diagnosis for these diseases between Australia and the United States (for example, higher incidence of chest x-rays and the use of bronchodilators in infants in the United States). We have confidence that when these differences are accounted for, we should see similar performance of our algorithms in the United States to that achieved in our Australian studies.