The reason that the total spike count decreases with increasing frequency is related to the lowpass filtering property of the RC membrane. For the same stimulus amplitude, a higher frequency stimulus results in a smaller membrane voltage swing...this is most pronounced for stimulus frequencies above the "knee" of the gain curve which occurs at a frequency f = 1/(2*pi*Tau_m). In this case Tau_m is 10 msec and the "knee" occurs near f = 16 Hz.