Incremental collections use G1MonitoringScope to record the result of a collection for JMX (MemoryPools, etc.). See the code in G1CollectedHeap::do_collection_pause_at_safepoint_helper(). There is no equivalent for concurrent collections, so memory freed during a concurrent cycle is not recorded for JMX if the concurrent cycle is not followed by one or more mixed collections. This can happen if the concurrent cycle freed enough memory to avoid having to do mixed collections.