How Discrete sequence could have more than one Frequency
When you are working with discrete sequences, frequency really becomes a different concept that "real world" frequency.  You can use analogy to relate information from discrete analysis back to the real world, but the two worlds are different.

First, once you have converted a real time / continuous signal to a discrete version, you immediately change the data representation such that no frequencies above 1/2 the sampling rate can be represented.

Second, the concept of frequency in the discrete world, with regard to the FFT, is based on the Discrete Fourier Transform (DFT).  The DFT imposes a restriction on your analysis.  The DFT assumes that all data is periodic.  The longest period represented in the transform is based on the length of samples being analyzed.  The representation says that your data is composed of a finite number of evenly spaced (in frequency) periodic sequences.  The set of periodic sequences are thought of as frequency bins.  The number of frequency bins is equal to to the length of the data.  So if you have say 8 samples your are working with, there are 8 frequency bins.  Half of the frequency bins are "reflections" of each other (represent the same frequency) so you are now down to 4 bins.  Two bins fall into the DC component, so you are left with 3 periodic sequences to represent your data.  So in terms of the original random signal you are analyzing, by doing an FFT analysis, you converted the signal from random to a band limited periodic signal composed of a finite number of frequencies.  This is very different from a real world random signal.

In your specific case, where you have computed fft([40 2 8 31 6 8 -4]), I don't know the mechanics of the specific FFT algorithm, but it looks like the data has been padded up to 8 samples, so you have a DC component and 3 additional periodic sequences.  Notice that the values ranging from 2 to 7 are symmetric around 4.5, so your periodic sequences are for frequencies represented by bins 2, 3 and 4 (5 is a reflection of 4, 6 is a reflection of 3 and 7 is a reflection of 2).  Bin 1 would be considered the period of the overall sequence.

You have to consider the sampling rate to convert the bins back to real frequencies: 

bin# * SampleRate / NumberOfBins