If you see the hard disk light up then your computer is swapping. Accurate results can only be obtained if the computer is not swapping. The benchmark is multithreaded and the calculations are correct. The issue is the bus that connects the CPUs. The number quoted by the manufacturer is the best case sequential read throughput.
Most programs read, compute and write back data rather than just read data.
Sandra creates 3 large arrays and performs various simple arithmetical computations on them - thus reading and writing memory. We feel this is a more objective test which measures the real throughput of the system. Of course, the cache does have an effect - unless it is turned off. My AMD Athlon gets a very low memory index. In most cases the caches also run in Write-Through mode, which slows down writes to memory appreciably when there are many such requests.