The fractional part isn't helping them serve data any faster. To the contrary, it actually reduces the speed from parallelism. E.g. a 5:9 scheme only achieves 1.8x throughput, whereas straight-up triple redundancy would achieve 3x.
It just saves AWS money is all, by achieving greater redundancy with less disk usage.
It just saves AWS money is all, by achieving greater redundancy with less disk usage.