NPR's Robert Krulich on how one word can have many many different spellings.
Mackerel, by the way, was originally an old French word, "maquerel" but when the Normans conquered the British (or was it the Angles and Saxons?) in 1066, they brought maquerel into the English language where it got bounced around quite a bit. James Gleick, the science writer, says the OED in 1989 listed 19 different mackerel spellings down through English-speaking history.
Nineteen is a lot different names to call a fish.
But when you consider that most people were illiterate, there were no dictionaries, and no notion of "correct" spellings, it would have been highly unusual for only one spelling to predominate.
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In its 2002 edition, The Oxford English Dictionary almost doubled the number of ancient mackerel spellings. Why?
Blame the internet.
After 1989 the internet made it easier for historians and antiquarians to scan, publish and exchange old documents. With a bigger database, all kinds of new/old spellings turned up.