It says that this percentage of reviews are within 1/2 disc of the DGCR average:
My conclusions are, either:
1) TR's do not stray far from the published average
2) Non-TR's take their cues on rating from TR's (*unlikely)
or
3) TR's ratings are no more meaningful than the DGCR average
*If most courses had lots of TR reviews, this could be a valid conclusion. But, I would TR's are in the solid minority of reviewers for most courses (especially early on while a course gets its average built).
I still maintain that the numbers would look very different if you looked at the rest of the data. I would have guessed before you started this project that you'd see the least variation at the top of the scale between TRs and the average.