I'm not sure. Same with Moslem or Muslim.
I wonder if its regional. Definitely has its roots in the fact that we translated them from a different alphabet.
But it's usually pronounced about the same.

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"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside