btrfs scrub complete

This was the status at the end of the scrub:

[root@supermario ~]# /usr/sbin/btrfs scrub start -Bd /media/Photos/
scrub device /dev/sdd1 (id 1) done
 scrub started at Tue Mar 21 17:18:13 2017 and finished after 05:49:29
 total bytes scrubbed: 2.31TiB with 0 errors
scrub device /dev/sda1 (id 2) done
 scrub started at Tue Mar 21 17:18:13 2017 and finished after 05:20:56
 total bytes scrubbed: 2.31TiB with 0 errors

I’m a bit perplexed at this information. Since this is a RAID1, I would expect it to be comparing info between disks - is this not so? If not, why? Because I would have expected both disks to end at the same time. Also, interesting to note that the 1TB/hr stopped being the case at some point.

• Btrfs
• Scrub