One makes the K sound before the S (Asterix, which could just as easily be spelled Asteriks) and one makes the S sound before the K (asterisk). Very easy to jumble up, especially for people who speak quickly. Since our language has so many words that end in the KS sound compared to the SK sound, it can feel more natural to say it that way. Sort of like how Wednesday (pronounced wed-ens-day) came to be widely accepted as wends-day because the D and the N together are easy sounds to mix up given that many tongues find it easier to put the N sound before the D.

Note that I have no science whatsoever to back up any of that, and fully expect Tim to show up and correct me.