These multi-threaded benchmarks can be very sensitive to their environment, you should try running it with nice -20 and do multiple passes to get a vague idea of the variability in the result. Also, it's important to minimise the number of other running processes.
I did not use the nice parameter but I always ran them multiple times and choose the average time. My system has very few running processes, minimalist ArchLinux with Xfce4 so I don't think the running processes are affecting in any way my tests.
