"Are acoustical lab tests really objective? If a company is paying for the test, don’t they just get the test results they want?"
An independent lab lives or dies based on objectivity. Any given lab handles hundreds of tests annually, and these tests are frequently called into lawsuits.
So when a lab lacks objectivity, they lose all credibility. At worst, they lose their certification is and the lab is out of business. And, since it's illegal to tamper with tests and test results, someone may even face jail time.
And, while competent material testing can be informally conducted by a manufacturer’s own lab, no one outside of that company will give it much credibility.
Where does good soundproofing data come from?
In general, objective data comes from a National Voluntary Laboratory Accreditation Program (NVLAP)-certified acoustics laboratory.
The labs themselves are closely governed by the NVLAP and must comply with a strict set of standards. Daily logs of environmental conditions, maintenance logs of all equipment maintenance, calibrations and more must be on hand at all times. NVLAP routinely inspects labs for compliance.
The test procedures themselves are highly scripted and defined by another governing body, the American Society for Testing and Materials (ASTM). The ASTM describes exactly how every test will be conducted, so that the results can be reproduced in another lab.
There is an ASTM test standard for virtually everything, far beyond acoustics. If you want the shear strength of a bolt tested, there is an ASTM standard test for that. If you want to test an air conditioner, there is a standardized test for that as well.
In summary, an independent NVLAP certified test is completely objective. Always trust this data over company-supplied "test results."