3 mirrored drives have significantly different used storage

I currently use two external drives and a NAS to maintain three instances of my backups. I noticed that each one has a significantly different amount of free space remaining: the NAS has 660.3 GB remaining, one of the drives (exfat filesystem) has 573.4 GB remaining, and the other drive (fuse filesystem) has 114.1 GB remaining.

All of these were just mirrored using FreeFileSync, and there are no differences flagged when comparing any of the drives to each other. I'm hoping this is a noobie mistake/misunderstanding as it prevents me from knowing A. If any of my drives are failing/hold corrupted data or B. How much storage I have remaining to use across each drive.

Thanks a ton!