Renji Overpower

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Hey guys, i've been wondering about this in the last 2 mangas: How could Renji become stronger that 2 experient Captains, on only a small space of time of training ? i Know, he was training on the famous 0 division, but i don't see logic in he becoming much more stronger than 2 captains with much more experience in battle, regardless of where he trained. What do ya Think ?