It seems we all may be missing the "backed" part - which I did on my first read through. They don't seem to be revealing how much of the logical volume is actual SSD, which I think is why they're instead putting down IOPS numbers.
Still, a huge and significant improvement over anything previously available. I'm looking forward to playing with it.
It's very likely that the entire volume is SSD, otherwise the tp99 would be atrocious (and if you look at the netflix numbers its actually quite good). The reason for the broad ranges is more likely that the SSDs have some inherent performance variability due to wear leveling and GC processes. Additionally, as time goes on, multi-tenancy will start to come into play (if Amazon has smaller SSD instances or larger machines), which will stay within that range comfortably.
Still, a huge and significant improvement over anything previously available. I'm looking forward to playing with it.