I remember reading somewhere a few years ago that if you took all of the PG players who were rated at showcases, most would fall in the 6-8 range, and I see that holds true. I know of some that actually flipped because they felt their son's rating should have been higher than an 8, looking back, thinking about his opportunities and those who recruited him and drafted low in HS, that was right on IMO.
Just our story, son's first PG rating came as a junior, 6 months later his second, although not performing as well as teh first time, he received the same rating. It was noted that he pitched just a few days before. That was fair. So doing well your first PG showcase is important, player should go prepared.
As far as ranking, closer to the draft, the stuff was pretty accurate from the predictions, I do beleive that Alan Simpson (who now works with PG) 10 picks away from his pick.
But good advice given to us was not to pay attention to that stuff and let the college coaches and scouts decide. I don't think that this is something we all have control over.