These names tend to be attached to them after the fact. I imagine there were a few Leonardos or Johannesses roaming about at their time so much like Alexander became The Great to set him apart from all other Alexanders, these names are scribes' and historians' shorthand to make clear which Leo or Joe you were talking about. And a few centuries of historical telephone later they seem to fit perfectly in our first name/last name system. Which in western Europe really only became officially standardized with the Code Civil from our friend Napoleon.