There are different ways of measuring volume (peak versus root mean square). Peak will take any point in a signal and display the current volume as its amplitude. Root mean square will take the square root of the average squares of a signal over a certain window. Peak will generally treat things as louder, though the signals may not actually be percieved as loud.

Thanks for this information

Though loudness meters use RMS and peaks, spectrum analyzers usually use Fourier transforms to measure the amounts of each frequency.

The spectrum analyzer in OP's media player doesn't look like it has the bin resolution of an FFT though, unless they combined bins.

Right, I forgot that the FFT sort of has an inbuilt lag to it (especially if using larger windows). RMS could still apply if you're right about the bin combination, though :P