Well look at their power differences already. We know Ulquiorra can dominate Yami, and Grimmjow is obviously doubting of his strength with only 2 ranks apart. No I think Abdula is along the right track, maybe the power differences aren't as great as in the 13 squads, but there is a decisive power difference, such as between Aizen and Toushirou, or Yamamoto and Ukitake.