Board Thread:Naruto Discussions/@comment-1514283-20140829001753/@comment-1512913-20140830140358

The different rank may be an error in fact, but I like to think Kakashi's skill with it makes it "S rank" rather than an error. Again, they are the same techniques, even Guy says so. Kakashi didn't develop Raikiri from Chidori as an improved technique, he cut a lightning bolt with Chidori and that gave it the nickname Raikiri. That means Kakashi's Chidori already was "S rank" when he cut the bolt in case it isn't an error.