Because the top strand is meant to be set at what is considered stock height ( four foot) and if you dont put two strands between that and the top of the netting then it will not serve purpose, in as much as that cows will put their heads between the bottom strand and the netting to either rub or eat from the hedge / boundry.
so how come when they put an electric fence up for cattle its only one strand as you say at stock height ???