How are the Divergence Number and world line related?
In Steins;Gate there is an important thing called Attractor Field. An Attractor field is a number of world lines that may be slightly different from each other in small details, but provide the same end result. 

Each attractor field is "made of" a number of World lines, in some range. For example, the Alpha Attractor Field contains world lines with the divergence from 0% to 0.99%, the Beta Attractor has world lines from 1% to 1.99% and so on.

Thus, the divergence meter is not showing some relative value, but rather an absolute value which is used to determine the Attractor field that the current World Line belongs to. 

Source.