First off, these numbers likely reflect drives sold more than a year ago, when capacities were <64GB in common drives. Since it's a retailer, I would also assume they don't peddle out the high-end drives as much (at all?). With a small amount of GB to play with, the drives are likely being used as system drives with a swap file (as default) on the C:\ drive as well. After a year (or more) of use, it wouldn't surprise me to see some drives start to buckle. Wouldn't put it past these home users to still be running WinXP too, and thus not getting the Win7 benefit of sector alignment, etc, not to mention the imaging programs that are sometimes shipped with the drives (most disk imagine programs don't take alignment into account *cough* Acronis *cough*).
Without full info of model numbers (at the least), this data is, as stated, a peek, but far from useful. Other question would be: is the numbers shown the % vs other drives, or %vs drives-of-type sold? One would assume that the return-failure-rate would be % vs drives-of-type, but one can't be caught a fool because of an assumption.