ARCH_NUDGE_TO_CENTRE picks the first for a pillar whose graphic has bit 3 set (graphic 8), the second for one without (graphic 6), and the entry by the pillar's facing (GET_SPRITE_DIR). Graphic 8 has bit 2 clear, so it only ever looks up entries 0 (mirrored) and 2; graphic 6 has it set and only looks up 1 (mirrored) and 3. Each of those four gives the same answer -- mirrored: along U, not: along V -- and the other four are never used.