Originally Posted by Rev.Chuck

Friction is what keeps a seatpost from moving in the seattube. From Webster:friction, the force that resists relative motion between two bodies in contact.
You are using a clamp to increase the level of friction to the point the seatpost will not move, much like you press down on the brake pedal hard enough to push the pads into the rotor to completely stop it so you don't roll back into the car behind you. The condition is static but it is still friction.

There can be some very fine tolerances. I used to rebuild rotary swash plate piston pumps for hydraulic drives and the piston clearance is so tight if you warm one in your hand it will not fit in the pump.

A inside/outside caliper is(typically) not accurate enough to measure a seat post and esp. not the seattube. You need an outside mic and a snap guage.