Candor said:
Aizen's reaction was probably because he never heard of that, which raises a couple of questions considering the new info we got from the latest chapters; why was Hikifune promoted to the Royal Guard? What about the previous 5th RG? Why was Aizen really shocked when he heard a captain was promoted to the RG? I think that's where Isshin comes in, he probably got exiled from there and lost his powers for some reason, which might be related to the Quincies, and Aizen was probably shocked at that fact when he heard for the first time a captain being promoted.
The thing about Butterflizen is probably related to the Hogyouku, which is related to Urahara, who supposedly was in the Spirit Realm, which raises a lot of more questions, though I think we're getting closer for some more answers.
@Han-yuu
I'm not really fond of that theory, but Aizen was born powerful, which raises more questions about how and where he came from.
Yep, for Aizen the idea of that promotion most likely appeared unthinkable. We do now know why, but most likely its because of what he knows about that place and Royal Guards.
Both Aizen's power and knowledge can be explained by making him Shiba(if Isshin is Shiba too, as well as previous 5tg Royal Guard), It would explain Aizen's sword name, his overpoweredness, his familiarity with Isshin, his knowledge of royal realm, as well as him targeting Shiba Kaien and knowing Ichigo "since the day he was born".
