I have never seen a Benchmark Return the Same Score run after run, CPU or GPU.
 
I have a Batch File that I run between Benchmarks.
 
Del "C:\Users\%USERNAME%\AppData\Local\Temp\NVIDIA Corporation\NV_Cache\*.*" /Q
Del "C:\Users\%USERNAME%\AppData\Local\Temp\NvidiaLogging\*.*" /Q
Del "C:\Users\%USERNAME%\AppData\Local\Temp\Low\NVIDIA Corporation\NV_Cache\*.*" /Q
Del "C:\Users\%USERNAME%\AppData\Local\Temp\*.*" /Q
Del "C:\Windows\Temp\NVIDIA Corporation\NV_Cache\*.*" /Q
Del "C:\Temp\NVIDIA\ComputeCache\*.*" /Q
Del "C:\Windows\Temp\NvidiaLogging\*.*" /Q
 

Before you start

"In general, you should benchmark every device you test under the same conditions. For example, you should test every system in the same location, at room temperature, and away from direct sunlight and other heat sources.
The precision of Futuremark benchmarks scores is usually better than 3%. This means that running a benchmark repeatedly on a consistently performing system in a well-controlled environment will produce scores that fall within a 3% range.
Individual scores may occasionally fall outside the margin of error since the factors that influence the score cannot be completely controlled in a modern, multitasking operating system. There are also devices that simply do not offer consistent performance due to their design. In these cases, it is necessary to run the benchmark multiple times, and then take either an average or a mode of the results."
 
- https://www.futuremark.com/support/guides
 
Other than that maybe your overclocks aren't 100% stable.