what's the difference between to_lsn and last_lsn in xtrabackup_checkpoints file?

Does anyone know clearly what’s the difference between to_lsn and last_lsn in xtrabackup_checkpoints file?
Sometimes they are the same. Sometimes not.there the details from the follow picture。

photoid=45702

photoid=45703

does anybody know the reason and have solution to the problem,help!