it depends on the game. some 3d games does not need all the raw power available from the gpu to run optimally. in such a case the gpu will clock down itself properly to save power and reduce the core temperature. it's parts of the power management of the card but you can force the card to run at it's full clock at all time if you want to. the changes can be made via nvidia control panel.
anyway before playing around with control panel setting you should check if this behavior also happen in other games you play or you can simply run graphical test benchmark such as heaven unigine and see if the clocks stays that way