For non-fundamental particles, when light (electromagnetic beam) hits the particle, the beam diffracts at a certain angle and intensity, dependent on particle size. This is the basis of particle size measurements, and you can find more information about it on the following link...
I think you’ll find it’s because although it is plotted against temperature, the temperature is really a factor of time. The desorption rate isn’t 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...
Quantachrome in the UK also provide a range of particle size instruments:
http://www.quantachrome.co.uk/en/Particle_Size_Analysis_and_Measurement.asp
Their website also has some useful links to particle size research papers and a dictionary.