When laboratory third party sound testing first came on line, it was the drywall manufacturers who were quick to get their drywall tested. In doing so, they set the testing precedence of how wall assemblies were tested. Early testing focused on the drywall above all other parameters. Perimeter joints, adjoining connection points, different types of ceilings, or steel I-beams were never really considered, as the emphases was placed entirely on the wall itself.
Testing this way is still important for drywall manufacturers. This way you can compare one brand of drywall versus another. Focusing on the drywall is not necessarily a bad thing when you are trying to determine the STC value. However, it is important to note as-built wall assemblies are far different than what is sound tested in a laboratory.