In reply to:

What had initially struck me as odd was the change made to the root phoneme <mag> by two different suffixes. This is looking, after all at two variants of one word, not two completely different orthographies embodying general rules. Is this common - I can't think of parralels right now, but doubtless someone can help...

Another example which we're so used to we don't even notice is the difference in pronunciation between the noun house and its plural houses , where the s sound changes to a z sound between singular and plural

Bingley


Bingley