on some case perhaps this is true but that ultimately depends on whats being bought. I've only ever bought one cgs coin a 1893 proof half crown, CGS unc 82, which according to their chart should have been pf64 or better, but it only graded PF63 when i cross graded at PCGS. Yet now re-graded its auctioned for almost double the CGS price i paid ....
perhaps i shouldnt moan about them , keep on grading there