contextCSN values and MMR

HI! Are contextCSN values on all replicas really in sync if changes were correctly replicated? I've implemented a monitoring check used with normal MMR setup (OpenLDAP 2.4.35, own build on Debian Squeeze) which also checks the contextCSN values on all replicas compared by server-id. Sometimes we observe, even in isolated tests, that contextCSN values for a certain server-id differ for quite a while (up to hours) even though the changes coming from that server were definitely replicated to all other replicas. After a while the contextCSN values get suddenly updated. Unfortunately this does not always happen. Any hint is highly appreciated. Ciao, Michael.