Typically, an x.9 score means that a specific check failed to allow you to hit the x+1 score. In this instance, you were maxed out at 7.9 (assuming Windows 7) and dropped to 6.9. Although WEI is mocked for its simplistic scoring, the underlying benchmarking system is reasonably sound.
Run 'winsat diskformal' from an elevated command prompt to see the detailed scores for the sequential and random reads/writes and see what the breakdown of scores are.
For example, on my laptop with a HDD, I get:
> Disk Sequential 64.0 Read 90.09 MB/s 6.4
> Disk Random 16.0 Read 1.37 MB/s 3.7
> Responsiveness: Average IO Rate 2.36 ms/IO 6.8
> Responsiveness: Grouped IOs 17.09 units 3.9
> Responsiveness: Long IOs 14.17 units 7.1
> Responsiveness: Overall 242.18 units 6.2
> Responsiveness: PenaltyFactor 1.0