Inconsistent WEI

I have two W7x64s installed on the same machine but each OS is installed on its own separate physical hard drive. The WEI for #1 OS is 6.8/7.4/6.0/6.0/5.9 while that for #2 is 6.9/7.4/7.4/7.4/5.9.
I wonder why there are so much difference between the two for the graphic performances.
The two HDDs are not the same model but are very similar. The one for #1 OS is HitachiHDE721010SLA330 and another for #2 is Hitachi HDS721010CLA332. Both are 1TB/7200rpm/32MB but the second one is newer model.
Has the performance of the HDD anything to do with the graphic performance?

The second is better although that is high. My gaming rig received 6.5/7.4/7.6/7.6/5.9.

I have a bit similar issue, but the difference is not that big. The motherboard of my notebook has been recently replaced (due to a malfunction in the integrated card reader) with a similar one (at least in theory they were similar), and after the change the WEI scores of my graphics performance decreased. Previously, the WEI of my desktop/gaming performance were 6.7/6.7 and now they are 6.6/6.6 on the same hardware. I re-run the assessment a few times, but I always got the same scores. Ok, its not a big difference, but still a bit disturbing.