I don't know much about actuarial math but I think this number is for insurance policies more than anything else. It could be based on something like the rate of hardware failures they experience now amortized over a long period and many customers, and then adjusted to account for redundancy.

As a very simplified example, imagine they are expecting to lose 2 servers every day, this percentage might be the probability of those two servers storing the same exact object (and thus, losing it irretrievably).