User blog comment:B214/General Discussion/@comment-27083298-20160929165937/@comment-9595671-20160930081114

Yeah, I know for one thing the outfit Akeno wore in the anime is not the same one from the light novels, nor are Rias and Asia's disguises when spying on them. In fact I think Issei's original date outfit from the novels is what they used for his date with Raynare.

Rias' outfit during the party and Rossweisse's armor were a bit different, but not as much. In those two cases it was probably just easier for the animators to have them that way when they animated the scenes.