3 (2 MADD + 1 MUL) * 1300 Shader Clock * 240 SP ~ 933 GFlops
3 * 1389 * 240 SP ~ 1.0TFlops
3 * 1527 * 240 SP ~ 1.1TFlops
So if they were previously guesstimating 1.1TFlops I believe they had expected the GTX280's shader clock to be ~1500mhz... 1300mhz means they fell well short. Hopefully they'll have some OC potential, it'd be nice to break past 1389mhz shader and claim the 1TFlop price.