Disk usage discrepancy?

No, the forget-satellite with a --force flag will delete data right away. If you skipped the --force flag, data may remain and will not be removed and you likely would need to remove it manually, see Satellite info (Address, ID, Blobs folder, Hex) for folder names of the satellites in the blobs folder.

If you used a --force flag, but still has a discrepancy, then I suggest to follow this recommendation:

and make sure that the filewalker on start is not disabled (it’s enabled by default).