Because that's not how it works,
what happens is that the fuel consumption goes down. Because the speed you set is the important variable and charging comes second, the engines are mainly used for propulsion and only the remaining capacities beyond that are used for charging. This means that during charging engines run at less efficiency and at a higher than with full batteries and therefore fuel consumption goes up until the batteries are charged.
Depending on the mod you are using there is a button that gives you maximum range at your current speed. This will vary drastically depending on whether or not you are recharging at the time. Can't remember if the button exists in plain vanilla, but I think not.