Brand new engines are tight and require bedding in and loosening up a bit before they reach their full potential, apparently the way it's been run in has an effect on the performance of the engine too over it's lifespan.
The compression is effected during the life of the engine by wear in the cylinder bore as the piston rings wear they reduce it. The engine also can get coked up inside with carbon, some carbon helps it run better, like on the inside of the exhaust, excessive build-up restricts performance slighly.