Pete, I have no idea, maybe because they are based on the original tear drop design.
Here is one with the axle more to the middle:
http://tnttt.com/viewtopic.php?f=27&t=24959&hilit=StomperXJ%27s+Sawtooth+XL
Here is one with the axle more to the middle:
http://tnttt.com/viewtopic.php?f=27&t=24959&hilit=StomperXJ%27s+Sawtooth+XL