Where do you come up with 2/3 of device sales being iPhones? How do those "devices" compare to the "1 billion active devices"? Did the numbers come out at the same time? Does "devices" mean the same thing in both stats? How are "active" devices determined?
Further, your assumption that 2/3 of the devices must be iPhones is erroneous. People keep iPods and iPads far longer than iPhones, many people still replace their iPhones annually.
The simplest math we have is 1 billion sold over 6 generations or so, and plummeting market share in recent years. iPhone historical sales are front loaded since Android has taken over, so the majority of iPhones are older models. Given the lifespan of a cell phone in western markets (where Apple has the bulk of their sales), it stands to reason that the vast majority have been sent out to pasture in some form or another.