Speaking of synonyms, here's a quote from German Sesame Street: "Wieso, weshalb, warum? Wer nicht fragt bleibt dumm." ;-)
Google Translate says it's "Why, why, why? Anyone who does not ask remains stupid."
"Wieso", "weshalb" and "warum" are synonymous. So if you'd translate the Sesame Street theme you'd have exactly the problem Levi indicated, they all mean "why".
"how come" was given as a translation. Can "how so?" not work? Or would that be a different word?