The results should reflect a comparable difference between different computers but are not comparable themselves. 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 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.
The prefetching instructions make sure that the data is already in the cache when needed. Thus performance higher than total bandwidth is obtained. 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.