One knows by comparing any bit of information, fact, or opinion with reality. There is an objective world independent of a human knower. One has learned truth when his or her ideas correspond to external reality. Truth is a matter of external rather than internal relations. Truth is objective and pre-existent to the search for it.
Variability is the result of human error in comprehending it.
An example of the correspondence theory is Realism. Consistency Theory Truth is that which is consistent between our ideas or impressions about reality. Truth always suffers when refracted through the prism of the human mind. It's difficult to tell who is nearer to truth. When we judge who is nearer, that judgment is but still another idea.