Because, unlike headings, runways are always rounded off to the nearest
ten degrees.

I'd say that the fact that headings are equal to their value, whereas
runway designations are not (they are equal to roughly ten percent of
their value, with exceptions) is closer to the reason. Runways are not
merely "rounded off", rather, the last digit of their magnetic heading
is ruthlessly ripped away after the roundoff process has taken place.
And even then, some runways are given different names ("Sorry, 9 is
already taken, how about 8?") for convenience. After all, it's better
than having runway "nine all the way left", runway "nine sort of left",
runway "nine right, looks like rightmost but isn't" and "nine so far to
the right it looks like another airport tucked in the corner".

Jose
--
Money: what you need when you run out of brains.
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