Assuming this is referring to the field names Track Volume on the track list and REPLAYGAIN_TRACK_GAIN in Help's Track Properties & Tag Formats (since there's is no other tag nearer in name to Track Gain), then Track Volume on the track list / Now Playing list is displaying a wrong value when Target volume for leveling tracks is not at the default value of 89dB.Replay Gain implementations use a multi-phase approach: first track volumes are analyzed and the resulting coefficient is saved to the track's tag (Track gain).
e.g. with Target volume for leveling tracks at 83dB, Track Volume displays a value which is 6dB different from the actual REPLAYGAIN_TRACK_GAIN tag value in the file, as displayed by e.g. Mp3tag and Foobar2000.
For example, it displays -5.7dB for a true REPLAYGAIN_TRACK_GAIN tag value of +0.3dB: