My best r15 run at 4.6 was 696,and that was with 3000mhz ram. Without xmp on it was in the upper 500 range. So the difference between 2133-3000ram speed can be significant. Though your scores not bad, and cinebench is only part of the story. If your system is running well, I wouldn't read too much into scores.
Yeah, your RAM speeds are the issue. XMP wont work if the RAM is only rated at 2133. But dont worry, for the 6600k, RAM speeds dont have a big impact. So your cinebench score is not as high as it could be, but that wont impact most other applications.