It is impossible to determine how 'accurate' TPGs are because grading is inherently subjective, especially when counting the angels on the pinhead that differentiates between grades or between half-grades. It is all subjective.
I once tried to measure my cards for technical compliance with PSA standards to cherry pick the best possible grade. All I could think to do was to measure the cards as closely as possible. I put a metric ruler on 1980 Topps cards looking for evenness of borders and lack of tilt on the card fronts. I found one card that was almost perfectly centered [less than a millimeter variation in size of white borders side to side and top to bottom] and not tilted [less than a millimeter variation from one end of the colored band on the card to the other. Razor sharp, perfect printing, clean everywhere, no discernable flaws. It got a 9 from PSA. Why a 9 and not a 10? I have no idea. And just to spread the love around, I sent in this card to SGC and got a 98, again I have no idea why 98 and not 96 or why a 98 but not a 100:
There's no logic to it and no reasonable rational explanation. On that day, at that time, the card was a 10 but not a "10+" in the view of the grader(s) who looked at it.
As for people who wish to collect based on the subjective opinions of third parties, doesn't bother me as long as they respect the rights of others to not collect that way. When they start running down collectors who prefer their cards in slightly 'loved' condition for whatever reason, then we can talk about how they are being manipulated.