The way I see it, Shimabuku loved to tinker with things, and was doing it until he died. Nagle and Long brought back an Isshinryu that was different from the Isshinryu that came to the US later with men like Armstrong, Smith, and Advincula. Each person's Isshinryu would have been adapted to their body type as well, the smaller Advincula or the much larger Steve Armstrong. Repeated contact with Shimabuku would have allowed a person to view the changes over time as well. Which leads to my next point, I bet if you dug around long enough, you'd find at least one Isshinryu dojo that does a version of Seiunchin that is closer to Goju-ryu's, and another that starts Naifanchi going the opposite direction. I'm not sure if Isshinryu was every supposed to be a completely codified system, but that's another discussion.