Thanks for posting to PX Community @dgibbons 

PX caliculates the logins by considering the identify calls and the session initializations. The PX sessions are calculated based on application usage activity tracked by PX and are independent of your application's session settings or logins/logouts. A PX session starts with the first event tracked for a user and ends when 30 minutes of inactivity occurs (PX has not seen/tracked any activity for that user).  

The next event that PX tracks for this same user … whether it is 31 minutes from the last event or the very next hour or day … starts a new PX session.

Users can and will have multiple PX sessions per day depending on how often a 30 minute window of inactivity occurs for them.

We can get the data of PX session counts by using the “Session count” Widget This widget displays the total number of sessions that were initiated during the selected timeframe.

PX session "duration" is calculated using the elapsed time between the first tracked event for a user and the last tracked event for that user within a single continuous PX “session”.

Some PX session "durations" can be recorded in seconds or minutes or even hours depending on how active the user was while PX was tracking their events within a single PX session.

Also, We can get the PX session duration data through the “Average session duration” This widget displays the average session duration of all the sessions during the selected timeframe. 

Let me know if this helps!

Happy Px-ing! :) 

Thanks for the reply @Cheemala Shivani