I need help interpreting FFT diagram I need help with analyzing the computed FFT of my signal. I don't wanna bother by going into the details of how my signal is formed so I will just state which frequencies I know my signal has. I know that the fundamental frequencies of my signals are 1,325 Hz and 1,079 Hz.
After computing the FFT of my signal I got the following FFT diagram.

My first observation is that the fundamental frequencies I expect are not identical to the frequencies I calculated. Is that to be expected when doing FFT analysis of any signal? My expected frequencies are mentioned above, the similar frequencies to those are 1.28 ( close to 1,325 Hz) and 1.038 (close to 1,079)

Secondly, I thought that the fundamental frequencies in my signal should have the highest amplitudes from all other peaks. That is the case with the peak at 1.28 Hz but not the case with the peak at 1.038. What could be the reason for that?

Finally, I know that the other peaks represent the rest of the harmonics of my signal but I am not sure how to interpret them exactly? what do they exactly say about my signal?
Is that to be expected when doing FFT analysis of any signal?

No. Either your calculation or the implementation of your analysis is off.

Secondly, I thought that the fundamental frequencies in my signal should have the highest amplitudes from all other peaks.

Why did you think that? There are many signals where the fundamental is lower than the harmonics or even entirely absent.

Finally, I know that the other peaks represent the rest of the harmonics of my signal

These are not all harmonics of your expected frequencies. You have also peaks at 0.25Hz, 0.5Hz. Could also be noise spurs, intermodulation, etc.

what do they exactly say about my signal?

Without knowing what that signal actual is, it's impossible to even  guess.