It's due to power leakage.
Different cpu designs and manufacturing processes cause current to leak in tiny amounts from the circuits. This varies from chip to chip. The less leakage a chip has, the less power it requires to operate at a given frequency. However since it's leaking less power that power gets converted to heat, it has to go somewhere.
A cpu that leaks more power requires more current (because it's losing more, like pumping up a tyre with a puncture) to operate at the same frequency as the other cpu, and stays cooler because it's leaking the current and not converting it to heat.
...or something like that! LOL