The computer must be right. What's wrong with me today?

When I ride my bike, it's most usually on one of a few rail trails with which I'm quite familiar. I have a cyclocomputer (are they still called that?), a watch-sized device which connects to a magnetic sensor to observe how quickly the front wheel is spinning and over how long a time, and hence to determine how fast I'm moving and how far I've traveled. I like being able to use it to track my progress. Earlier this year, though, I found myself pedaling at what seemed like a usual pace, but watching numbers indicating that I was going noticeably slower than usual. The display had to be right, though - didn't it? - so had I fallen suddenly and unexpectedly away from shape?  Why had it become harder to maintain a normal speed? I found it concerning, until I finished my ride and saw that what I'd always thought of as 18 miles (and which a smartphone GPS and topo map still did) was shown instead as about 16.3 miles, and that my elapsed time for the route wasn't abnormal for me. Further investigation showed that the cyclocomputer's setting for tire circumference had somehow gotten reset to its default, which was smaller than that of my actual tire. Once I readjusted that, simple division raised my indicated speed by about 10% the next time I rode. I felt better about both my pace and my device once the accidental bias was gone. I wasn't actually traveling faster, but a more reassuring measurement made it feel that way.