Most solutions (with the exception of inventory) retain their 'in-use' status, even if a machine is retired. You only gain the license back if the machine is deleted. (There are some exceptions, primarily if you have Asset installed).
In addition, some solutions (Application Metering) will install on workstations, but by default, won't install on servers. Other issues you may see depend on what schedules you have set for agent installs.
In any case, the numbers can vary from solution to solution.