Small Micron Dimension Particles are easily responsible for creating the interferences referred to.
All Ambient Air in a Industrial Environment will have many types of Particulate of this dimension suspended in the air and able to contaminate a surface and embed in the LP Groove.
An area that has a heavy burden of Traffic or a Local Railway, where there is Production Facilities will also have to contend with excesses of Metal Particulate suspended in the Air.
Release Agents used on the LP during production are also most likely to have a Small Micron Dimension Particle suspended in the solution, this could also be a Metal Particle in a excessive quantity, depending on the siting of the production facility used.
LP's are 'not' made in a Laboratory Environment where Filtering of Contaminants is a vital control measure.
The location of a Production Facility and the Quality Control measures followed during the production are most likely a cause of various levels on contaminant collecting prior to packaging and sealing.