Scanning Sound Files for Frequency

I am measuring the frequency in HZ of a sound file. When I select all (ctrl A), I get a frequency of 74.9 Hz (average frequency, Blackmann-Harris, area frequency FFT). But when I scan the file in the frequency analysis window, I get a frequency of 584.9 HZ although there is not a single frequency of that size in the file. What does the scanning procedure do? Accumulating frequencies? By the way, the scanned values are more in the range of Audacity frequency analysis.