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.