Seanmul89
Your complaining about the mysteries of the back of the whizz wheel not the wind side. Simply to be shown how to operate the wheel, without first ensuring you understand what it is you are doing, is akin to learning a foreign language without being told what the words mean.
Briefly, the earths atmosphere is defined by the International Standard Atmosphere (ISA). For any given altitude ISA determines the density and temperature and provides this throughout the whole Troposphere. All instruments are manufactured according to this standard. Of course, in the real world, the atmosphere is rarely ever the same as ISA. Therefore the pressure instruments will be in error. The atmosphere's temperature for any given altitude has the largest effect on the density. You need therefore to correct the indicated errors of the pressure instruments by calculation. The aircraft flight manual/POH will include graphs or arrays to calculate fuel consumption, climb rates, optimal altitudes and range variables.
You should read up on the atmosphere and understand how this affects flight. Also refer to your text books for the construction and mechanisms of the aircraft pressure instruments. All will become much clearer once you have done this. Not understanding "indicated" and "true" and instrument errors leads to many problems in aircraft handling also but this too is often ignored by instructors.