Also, there are several protection mechanisms built into the 5000 series cards. As one clocks higher and higher it seems as if the card slows down - there is a wall.
You didnt mention heat levels on your card - 900/1200 is pretty high on that card on air.
Many overclockers up the voltage/clocks and see what is happening to the various benchmarks - not just one. As soon as they flatten they have reached the cards limit and either stop or back off a little. People on air will see those limits much sooner - air simply cannot get rid of that much heat fast enough - even with that fan at 100%