Track Thing Accuracy

This morning into work, I tested the Track Thing accuracy against my car's odometer. According to my car I drove 12.4 miles, and according to Track Thing I drove 12.68 miles. So, the iPhone application does show more than what it is supposed to. I did notice that sometimes even when sitting still, the milage did increase by .01 mile or two. So, I don't know what sort of algorithm is used by the application, but it does seem to be off (show too much).