This is a phenomenon known as 'comb filtering'. It occurs when a sound is picked up by two mics at the same time. Because of the very slight differeces in the timing of the sounds arriving at the microphones, when the sounds from each are mixed together one is slightly 'out of phase' compared to the other.
When this happens, some frequencies effectivly cancel each other out; you hear a 'thin' sound because some of the original sound is simply not there!

A picture of the frequency plot of the two added signals has deep notches in it. The appearance resembles a comb - hence the name. See figs 1 and 2 below.