I agree with Werby and Macho on the pressure, plus there is one more factor. As stated, the water pump builds pressure against the restriction of the thermostat. Back in the old days of top tank radiators, low pressure caps, and no radiator overflow recovery system, if guys removed the thermostat, what was the next restriction? It was the radiator core. So at higher RPM, the pump is building enough pressure in the top tank of the radiator to push some of the coolant past the pressure cap. Less coolant = higher temps = the old, erroneous theory.