i.e. Does an 8 actually get D1 attention? Does a 6 actually get to play in college?
CaCo3,
To specifically answer your questions, my observations align with Bucsfan. And be sure to watch the video link that Sluggerdad provided. I think those two data points will best give you the info you are looking for.
In the video, as usual, Jerry admits occasional misses but understates how right they are with the vast majority. I think the misses occur when players are particularly young and haven't developed near their potential yet. I think most 8's would be borderline or not quite major or mid-major D1 potentially. A 6 will have a great deal of work and development ahead if he hopes to see a college field unless it's a really weak college program. That said, some players develop a bit more or less than expected in their late HS and early college years. Some work particularly harder or particularly less toward achieving their potential.
I believe PGStaff has stated that it is fairly common to either take down or not post scores for players who rate fairly low (probably in the 5 to 6.5 range).
The grade scale, I believe, is generously slanted toward optimistic potential.
My perspective comes from having regularly checked grades for a period of several years of those who have played for me as well as those who played on showcase and scout teams with son and with others I follow.