See the discussion here.

When the metadata stored in a local audio file is compared with the corresponding data on MusicBrainz, any discrepancies are indicated by a 'star' icon, and highlighted in the lower panel. This is useful as a prompt to update the local file by pulling the data from MusicBrainz and saving the updated file.

However, track times are different from other metadata in that they are a property of the audio itself on the local file, whereas on MusicBrainz they represent only an approximation to the actual track length (for example, they may have been copied from the CD booklet). So they cannot be updated locally in the same way; and although clicking 'Save' makes the star icon disappear, in fact it hasn't changed anything in the file – just replaced it with an identical copy with the same discrepancy which will show up next time it's opened.

It would be very helpful either to have an option in Picard that tells it to ignore these differences, or to indicate them with a distinct symbol.