@Garry H (Cut Nut) is best placed to explain this, but the simple explanation is (if I understand it correctly) lower HCA scores are for shallower stones, which may be at risk of 'excessive' obstruction/contrast when viewed in a ring (because the viewer's head is directly over the diamond, blocking the light and creating the obstruction/contrast, and in a shallower stone most of the light returned will be from the top). I think...??
Pendants and earrings are designed to be viewed face-on but hanging sideways, so the viewer is not obstructing most of the light entering the stone.
FWIW, I think my good lady's ring has a 0.9 HCA stone, and I think it looks great in all lights!