I never understood the goal of comparing iPhone sales to the S line. They are in two different universes. Apple has a captive audience. A massive portion of their user base is firmly entrenched in the iOS ecosystem and will not leave, either because they love iOS or they don't want to abandon all their purchases, etc. So when their only option, if they want a new phone, is to get the next iPhone, well, they'll buy an iPhone. And there is zero native competition to the iPhone. Apple has their 20% marketshare and can make billions selling phones to that slice.
Samsung, on the other hand, swims in a very densely populated pool that is the remaining 80%. While there might be a few peripheral things like Gear VR that might keep a user on the brand, there isn't really a big logistical roadblock... Someone with an S6 can move to a Nexus, or an LG, or Zenfone,etc... and bring their user, email, apps, etc. So the environment is a lot tougher from the get-go.