^ Yeah, probably somewhat from manufacture, but also from uneven use; rolling more than pitching, which I'm pretty sure we all do, or at least pulling more than pushing.
That will probably wear the four holes differently, and thus you have another mechanism producing preferred directions.
At first I also suspected that the orientation of the main spring (which is obviously cut at two places and therefore not symmetrical) could have a similar effect, but I tested it quite thoroughly the last time I greased mine and saw no evidence of that.
Another thing: if you mess with platform directions your calibration will most probably also end up out of whack, so make sure to check that.