Same thing has happened to my k/d so there's definitely a difference. I'm starting to think however that the difference doesn't come as much from levels or differences in attack or defence. I do just as well with my level 14 shingen-e as I do with my Hard Light. I think the increase in difficulty comes from all the best pvp people flocking to iron banner all day instead of worrying about any pve content. Also Since there is such a big reward for winning and barely anything for losing, you get people teaming up more, using communication, and tactics.