Just a stab in the dark here, but what about moving air?  Air blowing
from heating ducts, open windows and doors, walkways, etc., can effect
the output of organ pipes and blown bottles, particularly when operated
a low pressure.  Structures (casework, framing, brackets, supports,
etc.) near the mouth of the bottle can have more or less effect on the
volume of the sound as well as influence airflow around the mouth.

Adjacent bottles can modulate the sound as well.  There is an effect
in pipe organs [such that] when pipes are placed too close together,
the tone of adjacent pipes can change when sounded together.

Finally, maybe someone can enlighten us about the differences between
open pipes and closed pipes.  It is my understanding that in open pipes
sound comes from the mouth and the open top of the pipe, while closed
pipes (or bottles) the sound comes primarily from the mouth and is more
directional.

Bill Budenholzer