Time Machine backups use filesystem snapshots with reference-counted copy-on-write blocks. The total file size can easily surpass the actual amount of storage capacity. The same problem exists with modern virtual memory implementations, where the sum of allocated memory accessible to every process can be larger than the amount of physical RAM in the system (shared memory pages, file-backed pages etc.)
This is not about sanity checks. The value is correct. This is about presenting information in a way that makes sense to the user.
This is not about sanity checks. The value is correct. This is about presenting information in a way that makes sense to the user.