There are two different counter sources, volume and workload_volume. If you use qos statistics volume performance show -volume XXXXX -vserver XXXXX it should match. Volume is only at the WAFL level, but workload_volume (which AIQUM/OCUM use) is at the network layer (nblade) and WAFL layers. That may be your issue.
Also, the command could be having other issues (statistics volume show). As a workaround you could try nodeshell stats volume, then stats stop. If the output doesn't look right, please open a support case and we can identify the proper bug.