In AirWave reports, the Client Session Data is always restricted to a range of time (ie "1 month ago" until "now") that is defined in the report definition.
On the other hand, Client Inventory data can be restricted to a time range or you can report on all data. And in fact, the default value today is to report on all data. That time range restriction is governed by the "Limit to active devices" field in the Report Definition.
So, if you have a report whose Client Inventory data shows a greater device count than its Session data, make sure that in the report definition, "Limit to active devices" is set to "Active during report timeframe".