Math Easy Solutions, exactly — the volume layers make white-light replay easier because they act like a stack of insanely picky mirrors: they strongly reflect only the light hitting at the right color and angle, and mostly ignore the rest. That’s the Bragg condition: if white light contains many colors, the hologram “selects” the one(s) that match the recorded spacing, which is why a volume hologram can still reconstruct a clean image instead of washing out. Simpler version: a thin hologram is like one fuzzy pattern on a sheet, but a volume hologram is like many microscopic layers inside the material, so it has much stricter filtering and better image reconstruction under ordinary light, as explained in this Bragg/volume hologram overview.