Common Services lists all devices in DCR. By default RME will attempt to manage all devices in DCR (but this can be disabled). For devices which are in DCR, but not managed by RME, you can go to RME > Devices > Device Management > RME Devices to add them.

Put simply, RME's device list is a subset of Common Services'. While the two counts may be identical, it is possible for CS to have more devices than RME.