It moves because you have created a flow of air. The cavendish experiment works when the weights are separated by glass, plastic etc thus removing any current of air.
Try again.
You're never going remove air. It's always going to have an effect.
Don't pretend the experiment has been done in a vacuum.
Matter is constantly expanding and compressing dependent on energy applied to the area of that matter.
Any object displacing that matter will be acted upon by it's own compressive displacement of it and also the friction created by that displacement, which now occupies the surrounding area and now becoming a more compressed part of energy applied to it. This creates a more energetic expansion which acts right back onto the objects in question.
This type of stuff works with pendulums. You know: stuff like Foucault's and such like.
The only different is forced motion by energy.
I never mentioned a vacuum. I merely stated using a sheet of glass that would prevent air currents from moving the new mass. Again, you still can't explain why the small masses will always deflect towards the new larger masses and the displacement is always proportional to the mass.
You know Einstein's supposed warped space time?
Well forget about the fictional vacuum of outer space and concentrate on what we perceive as
a space.
Place a mass into it against another mass near to it and you warp that space with both masses.
Just one merest hint of molecular movement from the compression of these will cause a change in make up of atmosphere in that space, whether high or low pressure.
To make this much easier for you, Imagine you sitting in a bath looking sideways at yourself in a small mirror.
In gets fat belly Jones who displaces the bath water and also yourself in that bath water for that small amount of time. He created a change in the water movement.
It's always a knock on effect because EVERYTHING is attached with no real FREE space, anywhere.
If you weren't so backward you'd have a chance of understanding.