Synthetic benchmarks are just that, synthetic! lol they can not replicate real-world performance. For example, Furmark "burns in" GPUs, but it only uses 1 CPU core to do it (I regularly monitor temps, usage, etc). However, BF3 will use ALL eight of my cores while maxing out the GPUs.
In a scenario where 8 cores are being used instead of one, the real-world performance will be different. Not to mention the use of system files, memory, etc.