What makes you think it isn't the correct result? Have you checked you reproduce the answer for the sample case in the help for the function?
There's also another implementation of Haversine forumula on FEX I've used that seems to work reliably <Haversince FEX submission>; you can always check against its result for consistency.
And, of course, use coordinates for known locations and for which you know the correct answer to verify.
