Firstly I'm talking about TurboBoost, and sustaining the TurboBoost speed before throttling.
Better binned processors have better voltages and fewer leakages which allows the processor to produce less heat while running at a given frequency. This directly affects the temperature and TDP (not actually, TDP is a constant).
In fact higher frequency isn't the only constituent to more heat, but also the voltage and leakage.
That said, the above is based on my knowledge about overclocking, turbo boost might not work exactly the same. But Intel also mentioned this in their TurboBoost page:
Here it hints that the same model would also experience variances, which is normal. Better model = better bin = even better among the variance, unless they didn't bin the i7 at all.
That said, Kaby Lake is fabbed on a very mature 14nm process, i5 and i7 might effectively be on the same bin. This happened on desktop processors before, where Intel's fab process got so mature, even entry level i5s overclock to the same extent as top models.