RE: Why....
I agree Adrian.
The other thing is we gotta breath! As one gets excited and your heart rate goes up so does your breathing and this slight movement is one you can not stop.
I have noticed turkey behavior as I sat stone still but you could see they got nervous because of eye blinking. Another body function hard to control.
In a blind all this slight movement goes away. However in one of those blinds you need to watch how many windows you have open because you can silloutte your self because of the light coming through a window. It is better to keep it as dark as possible.
One way to check is take a friend and put them in the blind. Then walk around outside the blind to find the areas you should darken. A few of the blinds I see on the market the windows are far to big. Yes you can see better but it also makes it easier for the turkey to spot you or spot movement.
JW!