Spectrogram of a chirp and its FFT
FFT calculates spectrum over the whole duration of your signal (integrates energy at each frequency over the time of accumulation) while spectrogram shows frequency variation in time (instantaneous frequency). Divide the whole frequency band of interest in some equal intervals and evaluate how much time signal's frequency falls in each interval. These time periods aren't equal. So by FFT frequencies corresponding to larger time periods gains more power and will be major and vice versa. Now lets look at your spectrogram. We see that it's about 100 and 200 Hz when the most stationarity in frequency exists, there are maximums of the spectrum. Frequencies between 100 and 200 Hz appears 2 times, so we see raising of the spectrum at this span. Low and high frequencies appears only 1 time and frequency at these regions varies faster: the spectrum after FFT decreases.