Dual Core doesn't mean that you suddenly get twice the performance - ie 2x2ghz (as my laptop is) is not 4ghz.
Rather, you get the opportunity to share the load over two cores, meaning that should 1 app be single threaded, it can use one core in its entirety, while other processes use the other core. So that is why 2 cores will be faster than the same speed single core, even with single threaded apps.
Multithreaded apps are a whole different ballgame, but quite rare afaik.