Baseband FSK spectrum comparison Simulink & Excel

I have been using 2-FSK Modulator Baseband block along with the spectrum analyser to see how the spectrum changes as the simulation time, Ts is increased.

At present I have used 3 different Ts and therefore obtained 3 spectrum. If the information rate is Rb, i.e. bit period = Tb, then Ts = Tb, 2Tb and 3Tb.

The 3 Simulink spectrums are as per attached.

I have also derived equations for the spectrums and plotted results in Excel - results as per attached.

There is a noticeable difference between Excel and Simulink spectrums as Ts is increased. Why ?