hmmm.... i think you should specify that memtest also tests for vcore/fsb stability, specifically on tests 1-4. errors in these tests are usually fsb or vcore related, like too high for the former or too low for the latter.
tests 5 and above are geared towards stability of the memory (timings, vdimm).
people often mistake memtest as a "purely" memory related diagnostic. thats why there are many people complaining about pc3500s or pc4000s getting errors on memtest on oc'ed systems, even if they're running below or in spec.
hope this helps