[mcphage]

Another factor is, sometimes different forms of words have different pronunciations (because of the phonology of English), but often the same spelling: compare "electric" where the final "c" has a hard K sound, and "electricity", where he same "c" has an S sound. The pronunciation change is predictable, but the spelling retains continuity between the two pronunciations. It breaks the idea of 1-to-1 relationships between sounds and spellings, but in these kinds of situations, I think it's a good thing.