The best I can explain this is that the JKA changed a lot of their kata's. They changed niseishi and then called it nijushiho. They did the same with unsu and sochin and others. The reason for this I can guess is that they (the Japanese) wanted it to be separated as far as they could from Okinawan karate because the Japanese looked down on them. So they changed around all the moves and names. They did this around the late 40's and early to mid 50's. Some of the kata's that were added were sochin by Gigo Funakoshi, Unsu, nijushiho, gojushiho, were added by Nakayama when Gichin Funakoshi sent him and sometimes Isao Obata to learn kata's from Kenwa Mabuni. They brought these kata back and changed them or shotokanized them. I think Funakoshi talks about this in his book called Nyumon. The rest of this information I got from Sensei Nishiyama. As well as interviews done with Nakayama Sensei, who stated this. However, those kata were never officailly a part of the shotokan style. They were just experimental kata or extra kata. Funakoshi mentions this in his book and states that students should just spend more time on the 15 kata that Funakoshi learned and brought to the system.This is why the SKA under Oshima Sensei only practise those 15. I hope this helps.