Lee Enfields tend to have generous chambers, both in diameter and length of shoulder. Try an unfired round in the chamber, you will probably find there there is detectable looseness. Cartridges from different makers, or even production lots can vary in dimensions as well. Try measuring the diameters of a wide assortment of cartridges, and you will see variations.
When a round is loaded into a Lee, the extractor tends to push it to the left. This is why the firing pin strike is sometimes off-centre. Sometimes way off centre if a min. spec. round is fired in a max. spec. chamber.
When cartridge cases are drawn, casewall thickness may not be absolutely uniform all the way to the base. It may be thicker on one side than on the other. In fact, it frequently is. There are tools that can measure this, precision reloaders cull cases which lack case wall uniformity. When a round is fired, the thinner side will expand more than the thicker. This can result in the expansion ring just in front of the solid base showing more expansion on one side than on the other.