Yes, it's all rather confusing really, isn't it?
It really does seem to depend on:
The processor
The Multiplier that is set
BIOS
Motherboard
and all this in no particular order and without any real logic.
However, usually when overclocking you wouldn't want the Cool&Quiet feature, as this throttles the processor speed and voltages and the usual practice would be to disable this before overclocking. Maybe that is why it works with a minor O/C but doesn't when you go above a certain point.