Hi Eli,
a FFT algorithm computes the frequencies of the samples depending on the sample frequency. By changing the TD (time per division), the max frequency of the FFT changes. Try changing the TD to higher sample frequency.
The inherent nature of FFT algorithms is that frequencies above the Nyquist frequency (sample frequency divided by two) will be displayed as a lower frequency. That is the reason that ideally a low-pass filter should be used with a filter-frequency equal to the Nyquist freq.