I bike commute an identical route most of the time with few variations. Yet my daily mileage often varies by 0.1-0.2 mile over the 22 mile round trip. Why would this be? I am using a wired computer, so it's not due to signal interference that can sometimes affect wireless computers. I pump up my tires to the same pressure every morning before I leave. I travel the same roads because I don't have many other options. One-tenth of a mile might not seem like much, but if you roll it out on your bike it is pretty noticeable -- 528 feet. I can't believe that the actually distance I am riding varies by that much each day.

Any theories as to what could account for the variations?