Pieter,
are you sure you're looking at the right data ?
Could you post it so we can have a look.
The only possible explanation if the numbers are indeed correct would be the server/client not closing the connection properly with FIN handshake or RESET.
You would have to capture sniffer trace over a certain amount of time and verify all the connection close handshake and verify if the difference server/CSM increases or not during that time.
Do you have connection replication configured between active standby ?
Gilles.