Yes, the two key things: population of drives (model/version possibly, but ultimately not publicly known what the sample size is), and the rate is generally a m

Yes, the two key things: population of drives (model/version possibly, but ultimately not publicly known what the sample size is), and the rate is generally a maximum. i.e. use of "less than" symbol with the rate. Therefore, it's wrong to say every x bytes you should expect an unrecoverable read error. We have no idea how good the drives actually are, the reporting is in orders of magnitude anyway so it could be 8 or 9 times more reliable than the reported rate; or even multiple orders of magnitude. It's not untrue to say < 1 URE in 10^14 bits, but for the population to actually experience 1 URE in 10^16 bits. Why not advertise that? Well, maybe there's a product that costs a little more than advertises 1 URE in 10^15 bits. And other product classes that promise better.