But there is no #10 then. I'm saying that he occupies the numbers 0 and 10. The arrancar that came to Karakua with Grimmjow and talked to Hitsugaya told him that 11 and on are by birth order. The top 10 strongest are numbered 1-10 by strength. He knew that the top 10 were given the 10 least numbers. He just didn't know that it started at
0 and went to 10 with one espada occupying 2 spots, 0 and 10.
And another thing that I remembered was that when Ulquiorra and Yammy went to Karakua and Ichigo cut off his arm and Urahara and Yourichi came, Uli said that Yammy "couldn't defeat them at his current LEVEL" which infers that he meant that 10 Yammy couldn't win...not necessarily 0 Yammy. If he thought that Yammy wasn't ever capable of beating them he might have said something like "you can't beat them at your level of power."
Just something to think about
