I honestly don't know for sure either, but I see the same thing from time to time. I have one pool that's a uniform collection of Hitachi disks off an internal LSI2008 baed controller that never has errors, and a pool that's a mismash of WD (some 512, some 4K) & Seagate 2TB drives in an external case attached to a SAS expander by way of an LSI1068 based controller with external cabling, and go figure, it almost always has some small quantity of data that gets repaired with no device level errors (running this scrub right now):