You need to use --si instead of -h if you want to compare with the dashboard. The Storj software uses SI measure units (as a disk manufacturers), not binary.
12462425408 = 12.46TB
So, looks like the dashboard shows the used space pretty close to what’s reported by the OS. The average used space, if you would peek the last fully reported day likely has something about 7TB of the confirmed used space. So, the difference is about 5.46TB of uncollected garbage (see When will "Uncollected Garbage" be deleted?).
The overused itself is happened likely when you reduced the allocation to 10TB instead of previous likely 12TB, am I correct?
If not, and you are always have had the 10TB as allocated, then I guess that used space reported by the node was way off until the node is restarted and the used-space-filewalker has been calculated the used space and updated the databases, so now the node is aware that it has an overused disk space.
When the database is not matching the actual usage on the disk, the node wouldn’t report to the satellites that it’s full. Thus it can accept traffic and store more than was allocated.
We have a precaution mechanism though - when the node detect that it has less than 5GB on the disk or in the allocation, it will notify the satellites that it’s full and all ingress would stop independently of the databases state.
The only way to correct the databases state is let it finish the used-space-filewalker. It’s started on every restart.