last_hardenlsn and last_received_lsn

I have two node always on, and the secondary replica is a sync-commit replica.

I am checking for the column of last_harden_lsn and last_received_lsn , I am wonder why last_harden_lsn is greater than  last_received_lsn, why they are not the same ?

select * from sys.dm_hadr_database_replica_states