In order to get statistics working, you should make sure that you have configured the Garbage Collector through Setup to run always.
The ISP Session Garbage Collector runs a query at a minute interval, and calculates statistics that ISP Session will fetch.

NameValueExplanation
 AvgCompressionRate 1Average Compression rate
 AvgSessionDuration 0Average length of current Sessions.
 AvgStateLength 87Average length in bytes of session states.
 CountExpired 0Sessions that were expired just before they got garbage collected.
 MaxStateLength 87Size of the 'biggest' Session.
 MinStateLength 87Size of smallest Session
 SessionCount 4Number of sessions currently active.