I agree with a lot of what has been posted
fouling is rough,..and therefore affects the rotational forces. The jacket will take the brunt of that force. As the bullet exits,....it will be out of balance,..etc etc etc. The resulting effect is what pilots call "yaw"
Also,..yes gas pressures are increased,..therefore changing both the harmonic frequency, and the muzzle velocity. This is why your accuracy falls off,..you are no longer in "the node" and need to change something in your mix,..and since the load and the rifle are the same,..it stands to reason the change needs to be a bore cleaning as a direct result of fouling.