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It has been recently reported that the electromagnetic properties of pillar microcavities present an intricate and unexpected behavior for diameters smaller than the resonant wavelength. For instance, the micropillar quality factors can reach values well in excess of the quality factor of the reference planar cavity. In this work, we explain the reason for this behavior and provide an in-depth analysis of the energy-transfer mechanism through distributed Bragg mirror with finite lengths. We show that, in addition to a classical backscattering via the evanescent fundamental Bloch mode, the reflection is also mediated by another Bloch mode, which propagates in the mirror without leakage. This Bloch mode plays a central role in the electromagnetic properties of pillar microcavities and has to be taken into account for the optimization of the micropillar parameters.