Uncertainty Vs Variability

Ever wonder the difference between Uncertainty and Variability. I read a paper "A survey of non-probabilistic uncertainty treatment in fea" and quoting directly from there

"The term variability covers the variation which is inherent to the modelled physical system or the environment under consideration ".

"Uncertainty is a potential deficiency in any phase or activity of the modelling process that is due to lack of knowledge."

These definitions are straightforward and understandable. However, they are not mutually exclusive.