Electricity flowing between a generator and a customer moves through all lines connecting the two, not only along the shortest distance between the two points. The intended route for the electricity to follow is called a "Contract Path," but in reality the laws of physics govern how much current flows on any given line.
In the example above, electricity dispatched from the generator to the customer can flow through all the lines connecting the two. When electricity flows along several paths to a given destination this is called a "Parallel Flow."
Parallel Flows are one reason transmission lines must be constantly monitored. The amount of current flowing on any given line fluctuates in response to changing conditions, any of which can alter the resistance in a line, and thus alter current flow -- remember, electricity follows the path of least resistance.