I'm pretty sure it's because you only have 4GB and WEI arbitrarily sets the max that you can score with 4GB or less at 5.9.
It does the same with HDD's. No matter how fast your HDD actually is, since it's not a SSD, it has a max of 5.9.
Point being, WEI is one of the most idiotic things ever created. Don't even worry about it.