Although the 302 Hz tone has not actually changed amplitude, the spectrum looks very different. The amplitude of the tone appears to leak into other harmonics. The effect can be made more obvious by expanding the x-axis. This is the same spectrum, but we now show only the first 100 harmonics. The steps of 4 Hz become obvious.
Leakage is a direct result of using a limited set of sampled data.
There are two approaches to improving the accuracy of the spectrum when leakage is a problem. Change the sample set to line up with the harmonic content. Modify the sampled values with a weighted window. The term window has a specific meaning when applied to samples used to create a Fourier spectrum. Consider the signal in the first figure.