The current displayed value of a Spotlight metric should be raising an alarm, but that alarm has not yet been reported.
There are two parts to this problem:
Spotlight refreshes the metric data it collects at a rate specified in Spotlight Options. In most cases (but not all) this is the foreground rate when the control is visible, or the background rate when the control is not visible. These refresh rates (which you can set) are usually set to different values, with foreground data being collected more often. Data Refresh Rates
Spotlight uses metric smoothing (averaging) to prevent the generation of spurious alarms. It does this by averaging the data that it collects over a specified period of time. Alarm smoothing Metrics Dialog
The combination of these two features means that an alarm is raised faster for metrics in the page you are viewing than it is for those in a page that is hidden.