Magnitude Spectrum, different magnitudes same amplitude
To add a bit to the previous answers: the FFT result is "binned", not continuous.  A plot of an FFT result often fools people by being smooth for large enough FFT lengths.  But the result is really more like a bar chart for a sequence of bins, rather than a line graph.

And the bins are not rectangular in shape (in terms of frequency response).  The nearer a spectral frequency peak is to the center of an FFT bin, the more accurate the magnitude result.  You can get a list of your FFT's bin centers by multiplying sample_rate/length_of_FFT by integers from zero up to half the length.  If a frequency peak is not at the exact center of an FFT result bin, the magnitude gets split among multiple bins, which takes height away from the closest peak bin.

In your example, some of your frequencies were closer to bin centers than others.