^^ Instant was designed by the team that looks after the user interface and Google's "look". They are separate from the search team and probably talk to them even less than the Adsense team talks to the search team.
When they designed it, all they probably thought of was how cool it looked. And it does look cool - but it also interrupts the searcher when they type in queries. Which means that the searcher may stop too soon, and go off on a tangent, and thus get a set of results they didn't actually want (though the results served up may tie perfectly with what the little bit that was typed in).
Google is not a "joined-up" company. All their departments appear to function autonomously.