The letter Kappa in the Greek alphabet isn't exactly like an English K, we tend to aspirate initial K's whereas the Greeks don't, making the sound more like half way between 'k' and hard 'g' (as an approximation compare the aspirated K of Kin with the unaspiarated K of skin' which may sound like kappa when talking quickly. [note this may also depend on accent as some English dialects routinely aspirate medial Ks]) The journey from Konstantinos to Kostas is pretty straightforward - and then once we know that Kappa may be transliterated with a hard English /g/ as an alternative to /k/ (neither being exactly right) all is explained.