It looks like you where in side view instead of front view. As Martin said, it does it along the X axis. This the center point is the y axis.
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Martin Bergwerf
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...and just to be clear: it's actually the Armature's X-Axis, not the World X-Axis, the Symmetrize is using, but in your case (and by default) these are the same:
Just saying this, because Rotating the Armature Object won't help here.
