The "Variable" activity is mostly meant for emulation purposes. Its "value" works differently from other activities which is why pinning the statistic to a dashboard does not work (which is a bug/oversight).
Building a Statistics Collector to track the value is simple though. Just sample the activity as an event and choose "On Change". Use "current" as the row value and add columns to store the time (Model.dateTime), value (read from event parameter) and possibly object (the row value, only needed if the collector should listen to multiple activities).
A statistic like "Rooms In Use" seems easier to get as the number of tokens in a zone or resource activity instead of a Variable though.
