Concern over drive failure rates

Our JackRabbit storage unit uses lots of hard disks. The larger unit uses 48 drives in a 5U rack mount chassis. We selected and used the Seagate 750 GB NL drives for the unit, giving it a whopping 36 TB fully configured, with absolutely industry leading performance, density, etc. This is not a JackRabbit commercial, we are proud of our little L. Flavigularis though …
My concern is drive failure rates.

When the unit was sited in our lab, we had 1 drive failure. Swap it out, replace it , no problem. The AFR on these drives is 0.73, so one drive out of 48 failing is already about 3x AFR.
Thats not the issue.
Shipped it to one customer, they had another failure.
Sold it to another customer, and they had 4 drive failures.
I am thinking that the frieght company may have had something to do with this, though I would like to hear from others what their measured (ballpark) AFR is.
One of the ways we handle service on this is to depot store (onsite at customer) additional drives. If the AFR is really not what the manufacturers claim, but is closer to 5-10%, I will feel relieved, and happily provide 5 depot drives for 48 in-use drives. This isn’t an issue, though I need to understand if our expectations on failure rates have been mis-set by the manufacturers.
On this machine, we now have 4 failed drives. All replaced. RAID6 with multiple hot spares, so no data was lost. But this is more than an 8% failure rate.
This is what worries me.
We have not seen such a high failure rate on drives. Anyone else seeing this?