With 100 sessions shared across 4 nodes (i.e. 25 active sessions per node), the delta manager reports 100 current sessions per node (25 active and the copies of the sessions from the other nodes). The back manager reports 25. For consistency it should report 50. The 25 active sessions plus the 25 (or so) that are backed up on that node.
This has been fixed for Tomcat 7 and the HTML Manager application enhanced to differentiate between Primary, Backup and Proxy sessions. I suspect the changes are too fundamental (Backup manager changed to included backup sessions in list of active sessions) to be back-ported but I have started a discussion on the dev list
The documentation has been fixed but code will not be changed in Tomcat 6 as the changes required are too invasive.