[misframer]

It's in the documentation [0].

    An Availability Zone is represented by a region code followed by a letter identifier;
    for example, us-east-1a. To ensure that resources are distributed across the Availability
    Zones for a region, we independently map Availability Zones to identifiers for each
    account. For example, your Availability Zone us-east-1a might not be the same location
    as us-east-1a for another account. There's no way for you to coordinate Availability
    Zones between accounts.

[0] http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-reg...