Cassandra: memtable_offheap_space_in_mb and memtable_cleanup_threshold

I use C* v2.1.2 do write only workload (into one column family) and experiment with offheap-objects memtable space.

I set parameters to:

memtable_offheap_space_in_mb = 51200  # 50Gb
memtable_cleanup_threshold = 0.99

and expect that flush will not be triggered until available memtable offheap space reaches ~50Gb. But flushes are triggered before that limit. System monitor shows that in used only ~16Gb at that moment (linux+jvm+heap+...).

Why such thing is happened?