for example, the wire you desined is 0.2micrometer, but after fabricated, actually it is 0.21micrometer,
besides wire width, many other parameter may be vary with you desin, such as doping concentration, transistor length, wire height, and so on
p-variance such as wire length, width, transistor dimensions, etc affect the performance of the circuit hence it is important to make the circuit less sensitive to it. The basic classification in digital is done as fast corner, typical, slow. you can visualize it as devices formed at the one edge of the wafer, center, and other edge.