Yes that is the reason, David....
The RelativeAltitude tag is present in XMP DJI section as seen in following where i printed metadata for your submitted image and one from a regular P4Pro image:
DJI_0002.jpg {'Exif/ApertureValue': '2.97085', 'Exif/DateTime': '2020:12:31 11:09:08', 'Exif/DateTimeOriginal': '2020:12:31 11:09:07', 'Exif/ExposureTime': '0.003125', 'Exif/FNumber': '2.8', 'Exif/FocalLength': '8.8', 'Exif/FocalLengthIn35mmFilm': '24', 'Exif/GPSAltitude': '63.95', 'Exif/GPSLatitude': '46.8079801666298', 'Exif/GPSLongitude': '-123.030213166692', 'Exif/ISOSpeedRatings': '120', 'Exif/Make': 'DJI', 'Exif/Model': 'FC6310', 'Exif/Orientation': '1', 'Exif/ShutterSpeedValue': '8.31288', 'Exif/Software': 'v01.04.1546', 'File/ImageHeight': '3648', 'File/ImageWidth': '5472', 'System/FileModifyDate': '2021:01:10 21:14:35', 'System/FileSize': '9197148'}
DJI_0096.JPG {'DJI/AbsoluteAltitude': '+1864.63', 'DJI/FlightPitchDegree': '-6.80', 'DJI/FlightRollDegree': '+1.30', 'DJI/FlightYawDegree': '-88.80', 'DJI/GimbalPitchDegree': '-75.00', 'DJI/GimbalRollDegree': '+0.00', 'DJI/GimbalYawDegree': '-88.80',
'DJI/RelativeAltitude': '+45.80', 'DJI/RtkFlag': '0', 'Exif/ApertureValue': '4.97', 'Exif/DateTime': '2020:03:03 12:04:33', 'Exif/DateTimeOriginal': '2020:03:03 12:03:49', 'Exif/ExposureTime': '0.0025', 'Exif/FNumber': '5.6', 'Exif/FocalLength': '8.8', 'Exif/FocalLengthIn35mmFilm': '24', 'Exif/GPSAltitude': '1864.627', 'Exif/GPSLatitude': '39.6232487777778', 'Exif/GPSLongitude': '-104.876807305556', 'Exif/ISOSpeedRatings': '100', 'Exif/Make': 'DJI', 'Exif/Model': 'FC6310', 'Exif/Orientation': '1', 'Exif/ShutterSpeedValue': '8.643', 'Exif/Software': 'v01.07.1641', 'File/ImageHeight': '3648', 'File/ImageWidth': '5472', 'System/FileModifyDate': '2020:11:13 00:53:30', 'System/FileSize': '8101127'}
To come back to use of this script. This comes from the fact that for DJI drones, the altitude reported from the GPS is quite off so the idea is:
1. set reference Z for all cameras to relative altitude using the script (read_altitude_from_DJI_meta.py);
2. add the take off ground altitude to all reference Z using
https://github.com/agisoft-llc/metashape-scripts/blob/master/src/add_altitude_to_reference.py.
This way your camera reference altitudes or Z will be much more realistic.
In your example image, reference altitude is set at 64 m flying at around 80 m AGL, then the real reference Z should be something like 80 m + ground altitude (40 m maybe?) = 120 m