The p-i-n junction structure that develops dynamically under an applied bias via electrochemical doping (ECD) is of key importance for the performance of light-emitting electrochemical cells (LECs). While the complex electronic and ionic processes that govern its transient formation have been extensively studied by both experiments and drift-diffusion modeling, less attention has been given to the steady-state junction as a function of voltage. Here we study the formed p-i-n structure of a polymer LEC at the steady state by measuring and analyzing its most distinctive feature: the current density-voltage-luminance characteristics. Unexpectedly, we find that the effective conductance of the p-i-n structure exhibits a positive correlation with the applied bias, a behavior not predicted by existing LEC drift-diffusion models. We attribute this discrepancy to the assumption in these models of a constant density of mobile ions. Hence, we present a modified model in which the ECD level - represented by the number of ions with which the organic semiconductor is doped - scales with the applied voltage, implying a voltage-dependent doping efficiency. We validate this hypothesis using electron spin resonance spectroscopy and drift-diffusion modeling, while additionally establishing that only a small fraction of the available ions in our system, which increases from 1% to 3% with increasing bias, contributes to the ECD and is necessary for efficient LEC operation. These findings not only provide fundamental insights into the operational mechanism of LECs but also have direct implications for the broader organic mixed ionic and electronic conductor community.