According to Microsoft on the Graphics score, "This measurement is based on video memory bandwidth (megabytes per second), so the higher the dedicated graphics memory in your video card, the better this score is likely to be. A 256-megabyte (MB) video card, for example, is almost certain to get a higher score than a 128-MB card."
If you don't really have a small amount of dedicated video RAM, try updating your drivers and re-run the assessment. Don't read too much into it though, because WEI isn't known to be the most accurate representation of performance.