You probably don't have "Automatically write XMP metadata" on for one. Are you working with the images in Bridge too? Or some application that might be updating the metadata outside of LR? There's a mismatch, and that's why you see the symbol, not a big deal. Since you just updated something, chances are, you want the over-ride the older metdata so you want to sync it with the newer data (or you can turn on the Auto write XMP but that may slow the show things down).