Apparently the irregular versions of the word are earlier and tend to be replaced by the regular version as time goes on. The more common or frequently used a word is, the more likely it is to retain the irregular form for longer. Rarely used words succumb to the regular form earlier in our language history. Words in which both forms are used are in the middle of this transition process.

That's as far as i remember from my English Language course at university, anyway.

Bagpuss