It would appear that GIA have replicated dealers lighting, at very close proximity, which favours diamonds with steeper crown angles and deeper pavilions. The use of a gray tray further contributes to deeper stone preferences. It appears that computer metrics for brightness and fire have little or no role to play, and GIA have no metric or measure for scintillation.

Additional References

http://www.diamondcut.gia.edu/06_estimating_a_cut_grade.html