A An electric guitar's wood is important in creating sustain and producing a "pleasant" tone. The effect of the wood on a guitar's un-plugged volume is significantly insignificant.
As for a fan mounted in wood, I guess it would depend on the shape of the wood. i.e. a big flat piece would...
Here in Scotland where we have orange street lighting, a town can cast a noticable red glow on the cloud in certain conditions. I have noticed this from my parents house which is a few miles out of town.
Is that a possible explanation?
Not sure I subscibe to the friction explanation above. The differences that I see are:
+ on a treadmill, you are not doing any work to accelarate your body mass, but that normally only takes a second or two in real life, so isn't significant.
+ on a treadmill, you will not experience any wind...