Lightroom stores all the metadata (data about the data) in its own database, but Lightroom can also store all of that metadata in the “sidecar”, the XMP file.

Lightroom compares the data in its database with the data from the sidecar. If the data is different from its own database, then you will get:

Lightroom Sidecar Problem

Lightroom: Problem with XMP sidecar

By pressing on the notice (circled in red), you will get:

Lightroom Resolve the Sidecar Problem

Lightroom Resolve the Sidecar Problem

The choice is which one will wins, the XMP sidecar or Lightroom’s database? You choose. There’s no “right” answer. Which one do you trust?

Why would that happen? Some program may have “touched” the XMP sidecar, a corrupted file, a Lightroom bug…