I think it depends who you ask. I doubt there is a official reason for it. I guess it depends on what kind of context they're used in.
I think a lot of people assume using a "K" makes it sound more serious, so that's why they use it. Then other's just use it to sound snobbish. Then there's probably some who actually define the words differently. And those that think it's the same, just different spelling. Not that magic of any form is easy to define, no matter how it's spelled.
I mean, if you ask a orthodox Catholic what magic is, the answer will be different from what any sort of wiccan think it is.
---------- Post added at 10:13 PM ---------- Previous post was at 10:06 PM ----------
Why should you believe me? If you choose not to that is up to you, however.
My views on the varied spellings of the word stems from meetings I had with old 'practitioners of the craft' that I had when I was learning the craft some forty years ago.
I believe that is the way you and many people think. Doesn't mean it's the only way to define it. Maybe it even happens to be the most common definition, but if it's one thing I've learned, it is that ideas/terms/phrases can be defined differently depending on the context, without one being more "right" than another.
Obviously, that varies too. If someone calls a chair a couch, we're allowed to object. But with a word like magic and magik, there's not exactly a common definition.
I'm not saying you shouldn't define it the way you do though. I hope it didn't come off that way. If it's common practise for you and others, that's fine, interesting even since I've been curious about the different spellings before.