Why first_lsn of full backup and 1st transaction log backup are same?
Why first_lsn of full backup and 1st transaction log backup are same?

Because Full backups never break the log chain.  The log backups contain a contiguous sequence of log records regardless of how many Full backups were taken.
The log records in the Full are only there to enable the database to be restored and recovered to the point-in-time at which the backup completed.