Variability in individuals
Most statistical data sets take the form of a data matrix in which different measurements are made from a collection of 'individuals' (e.g. people, plants, houses, bank accounts, etc.). These individuals are not identical so measurements made from them also vary from individual to individual.
Unavoidable variability in measurements
Even when the 'individuals' are very similar, the recorded measurements from them often vary due to:
For the above reasons, the measurements in a data set are likely to vary from individual to individual.
This is the origin of the term 'variables' for the columns of a data matrix.
Yield from tomato plants
The diagram below shows the weight of tomatoes (kg) harvested from 20 tomato plants grown from the same packet of seeds in pots with as similar as possible growing conditions.
Observe that the yield from the tomato plants varies considerably, due to differences in genetics, unavoidable small differences in the growing environment and other chance occurrences.
Click Another 20 plants to grow another 20 tomato plants.