Are you asking "Why if I run the same benchmark 20 times I get 20 different results all within 2% of each other"? If so then the answer is: There is a multitasking OS running in the background still. What it does and when it does it, is the factor that causes it to be inconsistent.
To remove that factor would require you to boot straight into the benchmark. And there is no way on this planet Apple will ever allow to happen.