Look at the bottom of each row of trapezia. The subjective contour is very weak or even non-existent in the top row, stronger in the middle row and very strong in the bottom row. One reason is that alignment of the edges of otherwise unrelated (high entropy) objects is unlikely except as a result of occlusion by a nearer surface. Alignment of an orderly set is much more likely to be attributed to their arrangement (low entropy contrast).