I wonder if it's the combination of the plastic fill tube and the dipstick rubber o-ring. Rubber and plastic can be kind of, er, "grippy" when sliding against each other. I also think there is a bit of lockwasher effect, as the o-ring distorts when being tightened. I've noticed that when loosening a particularly tight dipstick I seem to feel the rubber gripping a little and then letting go as it turns, kinda like when you slowly slide a wet finger along glass.I'm not convinced that people are doing this. I've gone to remove one that I've tightened and had a hard time removing it. I think there's some other physical process going on (other than heat expansion) that causes it to bind, because it stays bound long after the engine is cool.