Welcome to science buddies and thank you for your question. I'm not a guitar player, and therefore not entirely familiar with the different holding techniques. However, after looking into the project, my thought is that whichever technique allows the most vibration of the soundboard at an angle most directed at the microphone is going to register a higher amplitude. It seems to me that the folk style requires less "body" holding of the guitar than the classic style. Since the soundboard is the part that collects and amplifies the sound generated by the strings, the more free it is to vibrate the louder and fuller the sound is going to be. Also, the sound tends to be directional out the front of the instrument, therefore, the more it is pointed at your microphone the louder it will be. This is of course assuming you have played the same note with the same strum force, as much as possible, in all tests.
Hope this helps.
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