Thank you everyone, for your replies to this point. I think I'm going to take a picture and post it, seeing as it sounds like a unique case.

I'm not 100% it's true for the entire circumference of the wheel, but I first noticed it while it was spinning in my truing stand. I asked my 15-year-old son to take a look at it, since his eyes are better. I didn't tell him what to look for, just, "what do you notice," and he mentioned the same thing on his own. That's then I pulled it out of the stand and set it on the floor to test whether it would lean.

I'll follow up with a picture or two, hopefully this evening.

For the record, I'd be pleasantly surprised to learn that this apparent phenomenon is in fact an illusion and that I am wrong about it ;-)