The usual pattern to names like this (both place and personal) is that
human mouths are not unlike rock tumblers - roll something around in
them for a while and all the bumps and rough edges get worn down.

So my own surname, forex, goes from the original form (below) to the
more common English "Sinclair".  "St. John" becomes "Sinjin", and so on.
  Syllables get run together, consonants vanish, are replaced by softer
ones or even turn into vowels, etc.

(If you want another analogy, consider how signatures evolve from the
written-out name, with recognizable letters, to a scrawl/symbol.)

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Kelly St. Clair
xxxxxx@efn.org