It can be true for the used-space-filewalker, if you did not have a discrepancy in the used and showed as used before and if you do not have a side load (Chia mining, your own usage, etc.) on the same disk.
Yes. Set this option to true (or remove it):
If you use a config.yaml, this option would be
storage2.piece-scan-on-startup: false
so you need either set it to true or comment out, save the config and restart the node.