I think youíll find itís because although it is plotted against temperature, the temperature is really a factor of time. The desorption
rate isnít constant at each temperature, and the curve is dependent on the time spent at each previous temperature step.
So if you think of the graph as desorption rate against time (s) instead, then it become clear why the integration result isnít what you expect.
I hope that helps. Also, here is an article on choice of adsorptive which you may find interesting: