Grain within any lot (trailer load, bin, floor store, lorry, etc.) can be defined in terms of moisture content, specific weight, varietal purity, protein content and many others.
However, grains are not identical and grain sampling seeks to enable a representative assessment of the characteristics of the lot.
Any one sample from a single point in a grain lot would probably be unrepresentative of the characteristics of the total lot.
A sampling programme aims to take sufficient samples to reduce the variability and provide a representation of the total lot.
The more samples that are taken, the closer the average will be to accurately reflecting any characteristic.