Ever since I got my new suspension fork, I've noticed that for the same trip I've made many times before the trip mileage reading has consistently decreased by 2% or so. Am I simply riding in a straighter line, or is something else at work? The computer is the same one, and the battery (which I changed, assuming that it was almost dead) is fresh.

The computer is simply counting the number of times the wheel goes around and multiplying that times wheel/tire diameter. If the wheel diameter has not changed, then you are most likely riding a shorter distance.

I think I might have found the culprit. The magnet had drifted a bit relative to its original position. So it's probable that the "counter" (not sure what the right term for it is) ends up occasionally missing a wheel revolution. I say this b/c I noticed how the instantaneous speed would occasionally gyrate wildly when I knew I was going at a more or less constant speed.