My guess (and it's a terrible guess) is this may be about 4mo growth... and they might take the opp to retire older lower capacity drives and just keeping up the build-out.
That's a good point --but they didn't start with 500PB, when they first went to AWS they probably only used a few PB to begin with and grew "organically" over the years. But likely still had some contractual stuff for this kind of scenario.