you're measuring from the v-brake mounts? i'd bet you have disc brakes, though.
i'd also bet you found a lazy wheelbuilder. one who doesn't bother to use a dishing
tool...he just eyeballs it...."meh, close enough." Since the hub has disc mounts, you
won't be using v-brakes, so he thinks it doesn't need to line up perfectly.

***never mind then....i had a similar problem a few months ago. dude that made
the wheels (disc hub, but using v-brakes) didn't bother to use a dishing tool to
center the rim on the hub. wheel was about 1cm to one side, when reversed it
was then about 1cm to the opposite side.

was the wheel properly centered in your suspension fork?