You probably can't rule out the possibility that you're subtly deformed in some way your parents hid from you, but I'm inclined to think it's the slope of the road near the shoulder. I noticed the same thing 20 years or so ago after I realized my tires wore assymetrically--the left side of the centerline wore out faster than the right. when I tried to figure that out, I realized I was seeing more of the right side of the bike than the left. I was sure there was something wrong with me or the bike. Turned out I lived where it rains a lot, and the roads apparently were sloped more than is customary. I solved the tire wear problem by reversing them on the rims whenever I happened to think of it. Either the problem went away when I moved to the desert, or I just quit noticing it. Haven't even thought of it in years. Didn't cause any trouble anyway.