Google doesn't like it when they are shown different content than a browsing user. This is roughly the equivalent of pointing Google Agent to a copy of the page requested that happens to be in Memcached instead of spinning up the full app stack to do the render.
Not a technically different page, specifically different content. Serving different pages to Google is fine, as long as they contain the same primary content that the real pages do. That's the whole point - so you can serve prerendered pages to Google but still have a JS-based frontend for the actual users.
AJAX sites often lazy load in content later. My point is the page delivered initially is not the same as the static version content wise or technically.
