After overclocking GPU getting worse benchmark results?

I overclocked my GPU AMD Radeon HD6670 2GB DDR3 - GPU 800 MHZ and Memory Clock - 800 MHZ.
I was overclocking with the MSI Afterburner. I can't change the voltage because it's not supported by the graphic card. I went to stress test it first at default clocks. After 1 hour the temp was 50°C (when idle 30°C) and I did the benchmark using MSI Kombustor and I got 777 points.
Then I went to overclock by 5 MHZ and did the stress test for 10 min. It seemed stable so went another 5 MHZ... I was increasing by 5 until I reached 860 MHZ and I got a system crash and I was like: what is a messed up something? But rebooted successfully and I decreased the GPU clock to the least highest stable clock (855 MHZ). I did the stress test for 30 min and no system crash so I decided it's stable.
But when I did the benchmark test I got 768 points .
And the question is why do I get lower results when overclocked? I don't think it's unstable.
Don't ask me about the memory clock because as soon as I move it to 805 MHZ I get a crash.
The temperature when at 855 MHZ was not going over 60°C when stress testing.

Thanks for reading.
Sorry for my English .

I don't think that card can be overclocked. With 855 MHZ GPU I get system crash every few hours, I just restored the default clocks. I don't think that it is supposed to work above 800 MHZ with 1.1V.
It's not throttling because GPU-Z shows 855 MHZ all the time when overclocked, however the Graphics cards gets downclocked when not used, from 800 MHZ to 400 MHZ and the voltage from 1.1V to 0.9V.
And no software allows me to change voltage, so the only way to do it is by flashing the bios but I don't want to void the warranty.