Calculation in HPLC

Hi everyone
As we well know we can make calculation by Area and hight instead each other and the result should be the same but a MOA which are USP Method the result are not the same and is is diffrent around 5% ,it is remarkable.
What reason behind this problem?

Peak height is never a preferred mode of calculation because it is not as reproducible as that if peak area as peak height depends upon peak broadening and peak sharpening and thus on column performance. It varies from run to run. Contrary to that peak area is more reliable as many system suitability parameters like resolution, peak width, plates etc are based on area counts.