Start PSA/
It's a good idea at the beginning of each ride to straddle the bike, lock the front brake and rock the bike a bit listening for noise from the headset. Any noise is a sign that the pre-load isn't right and should be attended to. Only takes 5 seconds to do the test and improper pre-load can affect handling and definitely puts additional stress on the steerer.
/End of PSA.

Your fork is safety critical.

What do you think would cause the headset adjustment to change between rides?

An improperly installed headset can gain slop in it as it settles in. Theoretically, it will also gain clearance as the bearing surfaces wears but not enough to be an issue.

i think the advice is good after a new headset install. A race that wasnt fully seated at installation could develop play.

Edit:
An improperly torqued stem (threadless) or poorly tightened locknut (threaded) could also cause loosening.

PSA: routinely check over your entire bike.

I think every bike I've owned developed play a few miles after installation. Not enough to be noticeable while riding, just barely detectable using the brake & rock trick. Yes, check your headset pre-load, especially after a fresh install.

As an old habit I drop/bounce the front wheel on any bike I get my hand on. I can hear a slightly loose headset a mile away. It rattles slightly instead of sounding solid. I verify with a brake grab and front wheel rock while holding my hands over the headset cap and frame to feel for movement.

Also helps diagnose loose wheel bearing adjustment. If the headset is tight but you heard a rattle then grab the rim and wiggle side to side to check for play.

In my experience, a threaded or threadless headset only has to have enough pre-load to hold everything together with no slop. I've repeatedly made the mistake of cranking them down too tightly, which only serves to make them notchy after only a few miles. Easy does it.