[jacquesm]

It's mostly because the voltage is assumed to be constant across the discharge profile (even if it isn't), and Ah is taken to the point that you can safely discharge the battery to. So you compare two batteries with the same voltage on their Ah, and systems in terms of Wh (and then the voltages are irrelevant).

One problem here is that what looks like a battery on the outside is often a system under the hood, so the output voltage is not necessarily the voltage at which the battery operates due to for instance DC-DC convertors and other electronics embedded in the battery housing.