Why Three FF Sensors?

Hi

Did I miss something, but I thought there were four FF Canon sensors:

18 1DX
20 6D
21 5Dii
22 5diii

Of course the 5dii must be due to end production, but Amazon are still selling them in the UK, so that makes it current in my books.

The OP's question is relevant though: why make a multitude of apparently similar specification FF sensors rather than one and use it across the range.

Regards

Chris