The mechanism of the tunnel electroresistance effect of a Co/PbTiO₃/La_0.7Sr_0.3MnO₃ multiferroic tunnel junction is studied in detail using experimental and theoretical methods. Based on experimental data, we present a model that explains the correlation between the polarization of the ferroelectric material and the observed resistance state based on the effective change of the tunnel barrier thickness.We show that the observed thickness variation can neither be completely attributed to the asymmetric inverse piezoelectric effect in the classical sense, nor to asymmetric screening of the polarization charge. The analysis of detailed ab initio calculations quantitatively demonstrates that a mixture of electronic and structural phenomena is responsible for the change in effective tunnel barrier thickness upon polarization reversal. On the one hand, the ferroelectric material exhibits a reversible metallization at one of the interfaces, which shifts the boundary between the ferroelectric material and the electrode. On the other hand, a piezoelectric effect that stems from different terminations of the ferroelectric ultrathin film towards the electrodes magnifies this effect. Combined, the electrically switchable effective change in thickness is as large as 0.15 nm, which dominates the resistive switching effect in the presented junction that involves a 3.2 nm thin PbTiO3 film. Thiswork contributes to the deeper understanding of fundamental mechanisms that lead to tunnel electroresistance and imposes new ways for tailoring the characteristics of electroresistive tunnel junctions.