Confusion about the relation between gibbs free energy and electromotive force
What that means is that $\Delta G$ is the maximum work available and therefore is only a thermodynamic limit. That's why $E$ is measured with a voltmeter where no current is flowing (actually extremely low current, i.e. the voltmeter has very high impedance). An analogy would be measuring gas pressure in a pressurized vessel, only in e-chem this is "electron pressure". Once you do work with an e-chem cell, the current will encounter many resistances which translate into voltage drop and in turn are energy losses and E will change.