I've noticed recently that most animes from 2002 to recent use different art styles during their OP and ED when compared to the main story. From a non-artistic standpoint I can understand that since the same OP or ED is used for a large number of episodes that they can put more details into them and other aspects without having to spend a lot of time on it. It's done once and used from then on. I also understand that some use Retro styles just because they can and it makes you laugh. This question is for the in between styles. They look similar, but it is obvious that it was done 'Differently'.
BUT the difference is sometimes so drastic that you can't even tell they belong together. Characters end up with such different looks that it takes you a moment to comprehend what is going on. While I do not find it a bad thing I feel that the work and detail put into the ED and OP could have also been placed into the story and made a world of difference.
My most current example: Chocotto Sister and its ED. It is a 2006 show, but the ED animation would have put it in the 2010 or 2011 category in my mind. It has that much of a difference.
(This question refers to OP and EDs with movement, not static images.) (Be aware, Chocotto Sister has topless scenes. Please be wary if reviewing an episode. NSFW)