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They tell you specifically how much RAM you get, and there is no confusion. They also tell you exactly how much storage you get. In this case they very specifically tell you disk read throughput.

So there are a lot of metrics that are not ambiguous. Imagine if they told you that you get a "moderate" amount of RAM, and maybe it could be 4.5GB, or maybe it's 9GB.

There would be benefits if Amazon gave SLA style promises for these sorts of instances. When they see that they are falling short, they redistribute loads or add hardware to meet the promise. Otherwise it's generally somewhat meaningless because if you can't plan on it, you can't build a plan around it.