Not sure exactly how it scores the HD's
I do know my WD Black Edition 1TB gets 5.9 as well as my WD 640 Blue.
As already stated seems that's the highest score you can get with a single Spinning drive.
To put things in perspective,
These drives on avg do somewhere at around 85-95 Mb/s getting 5.9 WEI
Some friends that run Raid 0 configs are pushing 150ish Mb/s (with similair 7200rpm drives) getting 6.3 WEI's.
(If i remember correctly, been awhile)
A single 30 OCZ vertex will do approx 200MB/s and WEI gives it a 7.3.
I know there's likely faster single SSds, but by the time you consider these guys in raid using 2 or 3 SSds, theres your 7.9.
Perhaps MS were just considering all of that ahead of time why those choose to do it that way ...
my guess is it somehow takes into acct the access time as well when generating a score. As thats quite a jump from a RAID 0 config, thats only 50MB/s slower. the only real difference is access time is near nothing on a SSD.