The intake air has a long path from the air filter to the combustion chamber. In between there are many joints and connecting points which are sealed by means of gasket or rubber hosing. If these become porous and/or leaky, then unfiltered air and contaminants is sucked in.
Contaminants can also be sucked in via inadequate filtration due to missing filter, defective filter or unsuitable filter.
Some people also replace the filter on the turbo inlet with a screen mesh. Although this improves airflow and is great for show, it's not so good for the longevity of the engine. The screen mesh cannot provide filtration of the level of which an air filter can.
Once the contamination reaches the combustion chamber it will increase wear on the cylinder running surfaces, pistons and piston rings. The reduced sealing of the piston rings to the cylinder surface then results in more oil creeping into the combustion chamber from oil sump.
Compression testing unfortunately does not show the worn cylinders and pistons, as the oil entering the combustion chamber helps provide a better seal and thus a higher compression.