The test applied at audit is narrower than the tests used for tax or employment law. Whether someone is an independent contractor under IRS or Department of Labor standards does not determine whether their payments are rateable — the certificate does.
A business can be entirely correct in how it classified and reported a worker and still be charged premium on what it paid them, because the questions being asked are different.
Why it matters at audit
This is routinely the largest single line in a premium audit. The full payment is added to payroll and rated at the hiring business's class code, not a fraction of it.