The documentation states:
https://docs.ceph.com/docs/mimic/rados/operations/monitoring/

The POOLS section of the output provides a list of pools and the notional usage of each pool. The output from this section DOES NOT reflect replicas, clones or snapshots. For example, if you store an object with 1MB of data, the notional usage will be 1MB, but the actual usage may be 2MB or more depending on the number of replicas, clones and snapshots.

However in our case we are clearly seeing the USAGE field multiplying the total object sizes to the number of replicas.

[root@blackmirror ~]# ceph df
RAW STORAGE:
    CLASS     SIZE       AVAIL      USED       RAW USED     %RAW USED
    hdd       80 TiB     34 TiB     46 TiB       46 TiB         58.10
    TOTAL     80 TiB     34 TiB     46 TiB       46 TiB         58.10

POOLS:
    POOL      ID     STORED      OBJECTS     USED        %USED     MAX AVAIL
    one        2      15 TiB       4.05M      46 TiB     68.32       7.2 TiB
    bench      5     250 MiB          67     250 MiB         0        22 TiB

[root@blackmirror ~]# rbd du -p one
NAME           PROVISIONED USED
...
<TOTAL>             20 TiB  15 TiB

This is causing several apps (including ceph dashboard) to display inaccurate percentages, because they calculate the total pool capacity as USED + MAX AVAIL, which in this case yields 53.2TB, which is way off. 7.2TB is about 13% of that, so we receive alarms and this is bugging us for quite some time now.