When configuring cdm probe Memory and CPU Usage alarm thresholds they are defined against the total usage metric, but this is not the case with the Disk Usage alarm thresholds.
Any currently supported version of the cdm probe
By default, the cdm probe collects disk usage metrics (in percent and/or MB) which keeps track of the amount of used disk space; however, the thresholds that you set in order to generate alarms for disk usage are actually set against the amount of free disk space that is available. This is not the case for either CPU or Memory metrics. The thresholds that you set for CPU and Memory Usage alarms are set against the total amount used. This is by design.