Right. In some set of slides, James Hamilton has certainly said at least how many buildings there are for us-east-1 and it's a lot more than the up to four zone

Right. In some set of slides, James Hamilton has certainly said at least how many buildings there are for us-east-1 and it's a lot more than the up to four zones people see (IIRC, >26). Most customers will never notice the randomization of building => zone, but every once in a while you rely on a vendor like MongoLabs and its important you both are speaking about the same us-east1-b. The AWS folks let you tie yourself together in case you have multiple projects that need identical mappings.