Board Thread:Questions and Answers/@comment-27127950-20151027213016/@comment-11051098-20160110001938

Dordoni talked to himself after Ichigo defeated him (I believe) about himself and his fellow Privarón Espada being demoted because Aizen started creating artificial Arrancar and thus stronger Espada.

Nnoitra talked to himself about being grateful to Aizen because he made Nnoitra stronger and able to enjoy fights more or something like that. I take it that he was created artificially due to his words, but it wasn't stated how long ago he and Nelliel were Espada at the same time.

Aizen's own, original Hogyoku could have been able to create stronger Arrancar for all we know, but being able to create even stronger ones once his and Urahara's Hogyokus had been fused together.