I will clarify that those numbers are for antibiotic prescriptions.
A lot of that comes, as you say, as a result of using antibiotics for viral infections. They are not going to respond. Those statistics come from different studies, and studies study slightly different things. The definition of “inappropriate” might be that it's just not the right kind of antibiotic for that particular infection. That would be one case. In some of the studies, it was that the duration of therapy was not right for that particular infection.
There are other definitions of “inappropriate”, but mostly it's about the mismatch between bacterial and viral, and using antibiotics for viral infections.