Theory of relativity
It's also a LOT easier to make a "no go" decision at some remote outstation as a charter pilot cause the boss isn't there saying "It's not too bad, I reckon you'll be right", or "it's not that heavy, you'll get off orright", or something such.
As an instructor, you may never have had to make that decision. How often do you fly the plane heavy, as Engine Out pointed out? Would you find yourself in deteriorating weather often enough to recognise it developing by merely what you see from on the ground?
I seem to recall when I learnt to fly that the weather didn't even have to be less than VMC for dual CPL level naxex's to be cancelled.
520.