A factory reset on the My Book Live re-creates the data partition's file system with mkfs. That writes a fresh superblock, fresh group descriptors, fresh inode tables and a fresh, empty root directory across the volume, and it does so in a pattern that touches a small fraction of the disk. The data blocks that held your files are not written; they are simply no longer referred to by anything. On an ext4 volume with a 64K block size, as the My Book Live used, the metadata that mkfs rewrote is scattered predictably across the disk, and the old metadata survives in the backup superblock groups the new format did not reuse.
That is why the units left alone recovered so well. The old inode tables, where they survived, give files their names, sizes and locations; where they did not, the files are carved from the data blocks by their contents and sorted by type. Every file written to the drive after the reset, and every backup job that helpfully ran, lands on blocks the old files occupied, and each one of those is a file that will not come back. The difference between a unit unplugged on 24 June 2021 and one used until August is the difference between most of it and some of it.
The bench images the drive, or both drives of a Duo, and works entirely on the image: the old ext4 structures are reconstructed from what the reset did not overwrite, the file tree rebuilt where the inode tables survive, and the rest carved from the free space. The 64K block size that defeats ordinary PC tools is not a problem on the bench, and nothing is written to the original.