Don't confuse the use of the Phoronix Test Suite and the Phoronix Test Suite itself. I assume you are talking about the reporting on results from particular runs of the Phoronix Test Suite.
Phoronix Test Suite is just a system to run tests in a repeated manner. If you keep the compiler consistent between kernels you are only testing the kernel. People usually raise issues when there are multiple variables changing between systems under tests (some say the kernel, some say the compiler, some say the filesystem).