mysql replication 5.1 ->5.5, time in "Seconds_Behind_Master", bug?

Question

MySQL

Good time of the day.
I set up mysql replication from 5.1 (master) to 5.5 (slave)
After the "start slave" command, I performed "show slave status" several times in a row and in the output I got:
1. Seconds_Behind_Master: 59858
2. Seconds_Behind_Master: 0
3. Seconds_Behind_Master: 58568
4. Seconds_Behind_Master: 57778
5. Seconds_Behind_Master: 57432
6. Seconds_Behind_Master: 0
7. Seconds_Behind_Master: 48545
8. Seconds_Behind_Master: 48054
9. Seconds_Behind_Master: 48054
10. Seconds_Behind_Master: 36 Seconds_Master: 0
11. Seconds_Behind_Master : 0 11. Seconds_Behind_Master: 0 1.1.
Further, the lag time did not jump, it dropped to 0, and it stays there, there are no errors.
I just can’t understand where 0 came from when the replica has not yet caught up with the master? Is this some kind of feature in the replica between versions, a bug in mysql or something else? Can the data in the replica be trusted? I would be glad if anyone can help, or tell me which way to google.
PS: if you compare "File, Position" on the master with "Master_Log_File, Exec_Master_Log_Pos" on the replica, then they match

Answer 1

well, there should be 0 in general ....