It could mean one of three things. There is some overlap between the possibilities.
If the session expired between the point where the list of sessions was generated and when it was displayed to you -1 will be displayed.
If the session has passed the TTL but the background thread that expires sessions has not yet run, you'll see a small negative number indicating the number of seconds since the session passed its expiration time.
If the session is configured to never expire (0 or negative TTL) then you'll see a negative number which is TTL - seconds since last access.
There is clearly some scope for clarifying which situation applies. Tomcat has all the necessary information to figure out which it is.