English has a set of words with "ch", coming — more or less directly — from the Greek language. They all have a /k/ sound.
Just to name a few. All these words are spelled with "ch" — no matter the pronunciation.
Then, there's autarchy. Pronounced with /k/, spelled with "ch"... and "k".
It might not be the only one, but I'm wondering why there's an odd one out in the first place?
Side note: I was about posting this question at https://english.stackexchange.com but then I realised that the same exception applies to my native language (German) as well. Autark is spelled with "k" (there's no "ch" spelling in German for this word); and all the other words I mentioned above are spelled with "ch". Psychologie and Archiv, however, are not even pronouned with an /k/-sound whatsoever.