Alright, calm down! It’s got nothing to do with spokes, superposition or anything like that.
There’s more than just one force acting on the system than your finger or the return spring. The seals acting upon the piston offer some friction and deformation against the piston. If they didn’t the system wouldn’t be able to self adjust.
As you said it’s a closed system once you’ve moved the lever a couple of mil. This is true irrespective of whether the pistons have adjusted or not. If the pistons just moved back and forth against a non elastic seal then they would never self adjust.
The adjustment comes because the piston seals grab the piston on the extremes of pressure or motion (forward and back) and allow some elasticity into the system to draw more fluid in when it’s open.
With regard to what I said, I should have made it clearer. The master cylinder spring does most of the work, but some is done by the deformed seal. When there’s a few thousand psi in the system the wet side of the seal is under the same pressure as the piston and deforms to move too. The bore is smooth but still lets the pressure through. That kind of pressure will cause the seal to deform and stick to the piston under deformation and draw it back as it relaxes. It’ll still have some friction against the piston when the master cylinder opens up, and if it’s moved at all (due to a differential in back-pressure compared to normal movement) then the elasticity of the seal will allow it to move out and draw more fluid through.