On this page, they note that:
Why is it different?The contrast with HERS is an interesting one. Two homes of different size are analogous to two cars with the same MPG that drive different distances. Valuing an upgrade depends on total energy consumption, not efficiency alone. Energy consumption is a function both of efficiency and use (i.e., house size, miles driven).
Unlike other existing rating systems such as HERS (Home Energy Rating System), EPS enables home buyers to assess actual energy consumption between homes, rather than comparing energy efficiency levels that could be similar for both a large and a small home.
Although the MPG metaphor is a catchy one, EAI is actually offering an assessment of energy consumption--which is the right thing to focus on.