Sometimes the names just come from different heritages, to use your Aaron example - there is also an Island in Scotland called Arran so that spelling is commenly used. Sometimes people just can't spell though.
For example take the name Keir - there are two legitimate traditional spellings - Keir which comes from the Scottish surname and Ciar which is gaelic and means Dark or Swarthy. Sometimes people will spell it as Kier which is taken as the original form of Kieran. Then you get all the adaptions of that Kyron, Kieron etc etc.