Originally Posted by
specq
most bike computers have a minimum speed at which they begin (and stop) registering. So if on some days you spend more time going very slowly up to or from a stop light or stop sign you're getting closer to your destination, but the computer isn't tracking it.
throw in minor path alterations (such as taking a curve wider than normal) and differences in tire pressure as mentioned above and I think you have your explanation
I'd suspect that this "minimum speed" is very low.
Keep in mind that computers aren't registering speed. They are registering wheel rotations. And one or two (a small handful) of rotations should be enough to cause the computer to start counting.