I'm guessing that's just core. Your fan speed will vary automatically unless you set it not to. Fans probably spinning faster to cool it down. Also I found that increasing the memory speed increases a greater temp increase.
I went from 650 to 810 (9600 GT), with just 5 degrees increase in temperature.
Power consumption (and hence heat dissipation) at full load is more or less linear to frequency (CMOS technology), but temperature depends on the cooling performance (at what thermal gradient can energy flows reach equilibrium). Thermodynamics is a complex subject.