We are talking about reference models and the reference specs are the official ones from NVidia. Don't think we can go into each and every card specs.
660 reference on paper is 1033mhz but what you actually get will be different.
Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.