Question

what is the true reason for variation​

Answer 1

$\huge\sf\large\large{Answer :-}$

Genotypic variations are caused by differences in number or structure of chromosomes or by differences in the genes carried by the chromosomes.

Answer 2

Some examples of common causes of variation are as follows: Poor product design, poor process design, unfit operation, unsuitable machine, untrained operators, inherent variability in incoming materials from vendor, lack of adequate supervision skills, poor lighting, poor temperature and humidity, vibration of machinery, inadequate maintenance of equipment, and inadequate environmental conditions due to noise and/or dust.