Most likely with one small spot like that is you had some contaminate on the wood before you glasses, was or oil most likely. If you used acetone to clean up sanding dust that may be it because most acetone now days is reclaimed and often has oil in it.

Truthfully I'd go with Jay's advice, ignore it, won't hurt anything.

If you do want to fix it, you are opening a can of worms, it's pretty easy to make it look worse, harder to make it look better.

Bill H.