how do you feel while doing the exercise? do you feel its moderate, heavy or light exercise?
i think if you are using a higher gear and seem to be taking less revolutions it might be towards the lower end of the scale - light. why not push yourself just that little bit more and reduce the level a couple of notches and aim to get 12kph for a change.
otherwise perhaps search some other calorie counting site for other descriptions and go from there - take an average. here is one to get you going
Counter. There is the option of light, moderate or heavy cycling.
i hope this helps.