Thanks for the reply, JJ.
(Coincidentally, the reason I started going through these old tags is I was looking for J.J. Jackson tracks, changing the spelling of his name - to remove the space between the Js. So it's kind of cool that you responded with help.)
Anyway, good question!
When I look at the Properties, it shows
Year: 1978
But when I look at the Extended Tags, it shows
YEAR 1978
YEAR 1978
It shows it twice, suggesting I have two YEAR fields.
Now I'm even MORE confused. I don't see how that could have happened. I certainly didn't type the year in twice, in all 2,800 tracks! Maybe somehow I accidentally duplicated the field in one action. I didn't even know it was possible to have two fields with the same name.