Performance variation in traditional HDDs?

We recently benchmarked some identical HDDs (same part number,
purchase date, and firmware) with fio and were surprised by speed
differences between them. Intrigued, we bought a few more sets of
identical drives and benchmarked them. It makes sense that there would
be some variation - after all, these are rotating platters and
physical rotation speed may vary a bit.

All sets of identical drives showed some variation, with some showing
significantly more than others. Note that when the exact same drive is
rebenched, it generally shows the same results. So different instances
of the same drive really do perform differently.

For example, we benched 6 of the same 2TB 7200 RPM drive and did a
full drive write with fio. The slowest drive averaged 158MB/s over the
run. The fastest averaged 174MB/s. And the average of all drives was
169MB/s.

I was wondering if anyone had pointers/thoughts on what kind of
variation should be expected in performance with HDDs, or
articles/research about this behavior?

Thanks,
Nick