...I am talking of the knowledge required that is required to attain the CCN1 and CEN1 and whatever else is required (CPA1?). Which parts of that are difficult to learn? ...
It's a piece of you know what. It has nothing to do with working on gas though. It is purely the basics of "working safely", and very little more.
A good comparison would be the theory part of your driving license; it's a legal requirement, but has little to do with how much you know about driving.